PDF Summary:On War, by Carl von Clausewitz

How is it possible that defenders are responsible for most wars? Why is it a logical contradiction for military concerns to influence political policy? And why shouldn’t soldiers sleep in tents? We’ll answer these questions and more in our guide to On War.

On War is arguably the most influential Western treatise on the subject of war. In this guide, we’ll discuss Carl von Clausewitz’s philosophical ideas about war and walk through the chronology of a hypothetical war as we examine his tactical and strategic advice. We’ll also compare his advice to that of other military strategists, such as Sun Tzu, who wrote The Art of War (arguably the most influential Eastern treatise on war), and also consider non-military applications of some of his ideas.

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(Shortform note: While housing troops in civilian homes along their route may have been convenient for the military, even in Clausewitz’s day, some people viewed it as an invasion of privacy and a burden on the civilian population. This sentiment is reflected in law: The English parliament prohibited housing troops in civilian homes without the voluntary consent of the homeowner in the 1680s. And the Third Amendment of the US Constitution similarly restricts quartering soldiers in civilian homes.)

Sourcing Food in Enemy Territory

Once you cross the border into Country B, you need to adjust your routine. You still get most of your supplies from the locals, but now you do so by sending soldiers to the local officials and demanding that they give you what you need. Clausewitz notes that this is not only the most efficient way of sourcing food and common supplies, but it also has the added benefit of weakening your enemy since you’re now feeding your troops on B’s resources instead of your own.

That said, Clausewitz also stresses that it’s important to move quickly through enemy territory because your army will exhaust all the local food supplies if you stay in one place too long. And if you run out of local supplies, bringing them all the way from your home base in wagons is risky and expensive.

To allow for unforeseen delays or emergencies, he recommends having every soldier carry four days’ rations in his own pack and keeping another four days worth of food for the whole army in supply wagons that move with the army. That way, if the need arises, you can travel or fight for up to eight days without any new supplies.

Living Off the Land

We could generalize the essence of Clausewitz’s advice about supplies in foreign territory by saying, “Travel light, so you can focus on your mission.” This general principle applies whether your mission is a military invasion, a business trip, or an extended vacation. As author and entrepreneur Tim Ferriss points out in The 4-Hour Work Week, the less luggage you have to carry and store on a trip, the better your travel experience will be.

If you’re traveling to another country for business or pleasure, you probably wouldn’t even think of bringing your own food with you—you’ll just buy food from local shops or restaurants. Just contemplating the hassle of bringing your own food drives home Clausewitz’s point about the challenge of shipping supplies from home, especially when you multiply your daily needs by tens or hundreds of thousands of men in an army.

Of course, when you travel for business or pleasure, the locals aren’t your enemies. But just as commandeering food from the locals in enemy territory contributes to a military goal of weakening the enemy, shopping locally while traveling contributes to your mutual benefit by supporting the local economy.

And the principle of relying on local supplies so you can travel light applies to more than just food. Instead of bringing everything you think you might need on a trip, Ferriss recommends just bringing some extra money to buy anything you find you actually need once you get there, like insect repellant or a raincoat. This cash reserve loosely corresponds to the limited supply of rations that Clausewitz advises having soldiers carry for emergencies because it allows you to deal with unforeseen needs as they arise.

Ferriss also recommends borrowing things from people who already have them. For example, if you want to go whitewater rafting, you don’t need a raft—you need a friend who already has a raft.

Lodging in Enemy Territory

Clausewitz says that if you’re confident that Country B’s army is too far away to threaten you with a sudden attack, you can still quarter your troops in local housing on foreign soil. This is an efficient way of living off the enemy’s resources.

(Shortform note: In business, an analogous tactic is making use of your competitors’ services to get people to use your product—initially in addition to your competitor’s product, and eventually instead of it. For example, as a startup, Airbnb made use of Craigslist (a competitor in peer-to-peer property listings) both to identify prospective hosts and to cross-advertise listings.)

However, as the likelihood of encountering B’s armed forces increases, he says you should transition to making camp each night. In camp, the army remains together instead of spreading out to shelter in different houses. This provides better security, since your night watchmen don’t have to patrol as large an area, and your soldiers can assemble for battle more quickly.

That said, even when you’re planning on making camp, Clausewitz advises against bringing tents for the soldiers to sleep in. This is because it takes many horses to carry enough tents for an army, and those horses could serve you better in the cavalry than in the supply train. He assures you that bivouacking (camping in the open air) won’t degrade your troops’ health or fighting ability.

Recreational Bivouacking

Today, bivouacking is gaining popularity with some recreational campers. Even with modern, lightweight tents that humans can easily carry, by eliminating the tent, bivouacking saves weight, bulk, and cost on camping trips.

It also allows you to make better use of natural shelter, such as overhanging rocks: You can sleep at the base of the rock, but you probably wouldn’t be able to pitch a tent there. Similarly, bivouacking gives you more options when camping in rough terrain. In steep, rocky, or densely forested areas, it’s easier to find a clear, flat spot just large enough to roll out your sleeping bag than it is to find a clear, flat area large enough to pitch a tent in.

There is also less evidence of your presence because tents are more visible than people sleeping on the ground, and tents often require driving tent stakes into the ground, leaving minor disturbances even after the tent is taken down. Clausewitz would probably be quick to point out that less visibility is great for added security if you’re camping in enemy territory. It also aligns well with the “leave no trace” ethic practiced by many modern outdoor recreationalists.

Of course, the downside to bivouacking is that it provides less shelter in the event of inclement weather. You can minimize this downside by checking the weather forecast and only camping out when the weather conditions are expected to be favorable. In Clausewitz’s day, most armies only fought during the summer months, probably at least partly for this reason.

Winning a Battle

Let’s say the first resistance you encounter is a chain of outposts guarding a river that you must cross on your way to B’s capital. As Clausewitz points out, the river limits how fast you can move your troops into position to attack the enemy forces because it takes time to build a bridge or ferry your troops across the river. This gives B’s defending forces an advantage.

However, he also says that, in this case, it’s not a major problem because your army significantly outnumbers the enemy force. Strength in numbers is more consequential than the advantage or disadvantage that terrain features like this afford. In particular, Clausewitz assures you that outnumbering your enemy by at least two-to-one practically guarantees victory in most cases.

Moreover, this situation actually amplifies your numerical advantage, because the defending force is strung out over a long stretch of river. They don’t know where you’ll try to cross the river, so they have to guard its whole length. But you can bring your whole force across at one place. This way, you’re using your whole force against a small portion of theirs.

Sun Tzu’s Perspective on Numbers and Terrain

Clausewitz and Sun Tzu agree that having more troops gives you an advantage in war, but Sun Tzu takes a more conservative stance on how big the difference should be. He prefers to have 10 times as many soldiers as his enemy so that he can completely surround them. If you have two to five times more troops than your enemy, he advises trying to draw the enemy into an ambush. And if you only have the same number of troops as your enemy, he says you should avoid battle.

By contrast, Clausewitz generally feels confident of victory with at least twice as many soldiers as the enemy. If he saw any additional advantages to outnumbering the enemy more than two-to-one, he doesn’t discuss them. And he seems perfectly comfortable engaging in battle with equal numbers or even being slightly outnumbered if he has other advantages that make up the difference, such as better equipment or better morale.

Like Clausewitz, Sun Tzu observes that terrain features such as water can provide an advantage to one side. In the case of rivers, he corroborates Clausewitz’s observation that crossing a river slows down an army. Sun Tzu goes on to advise that you can use this to your advantage by waiting for the enemy to cross and then attacking them with your whole force when only half of their force has crossed, effectively doubling your numerical advantage.

Using Reserve Troops to Reenergize Your Forces

Let’s say you ferry enough infantry across the river in boats to hold a position on the far bank while your army builds a makeshift bridge. The bridge allows you to bring the bulk of your force across the bridge before B’s whole defending force can converge on your location.

But, on Clausewitz’s advice, you don’t send all your troops into battle at once. Instead, you send just a few more than the enemy appears to be fielding and keep the rest far enough back to be out of the battle. Clausewitz calls the troops that you don’t initially send into battle your “reserve force,” and it can serve a number of functions. For one thing, there’s always some uncertainty about how a battle will unfold, so it’s good to have troops ready that you can send in to counter the enemy’s unforeseen moves.

Moreover, you can use them to reinforce and refocus your troops in combat: Clausewitz observes that battles often open with focused, coordinated fighting but quickly devolve into disorganized, reactive fighting because soldiers easily forget their plans and orders under the stress of combat. If you send in fresh troops after the fighting has become disorganized, they can help all your forces refocus on their battle objectives. If the enemy forces have become disorganized by this point as well (and don’t similarly refocus their forces with fresh troops), this gives you a huge advantage and usually results in a decisive victory.

(Shortform note: Sun Tzu also suggests holding back a few of your troops to counter your enemy’s unforeseen moves or exploit enemy weaknesses that become apparent later in the battle. And he discusses the importance of psychological factors in battle. But he doesn’t combine the two as Clausewitz does when he recommends sending in reserve troops to refocus your attack.)

Finally, when you win a battle, you can use your fresh troops to pursue the enemy.

The Einstellung Effect

Even when you’re not involved in a literal battle, it’s easy to lose sight of the big picture and stop making progress because of the way your brain works.

In A Mind for Numbers, educator Barbara Oakley explains that your brain normally alternates between two modes of operation: In focused mode, your mind is filled with one particular task or problem, and your synapses (neural connections that make up thoughts in your brain) are concentrated in a particular region of your brain. This mode is important for solving problems and executing tasks because it facilitates processing information in detail, and you have to keep track of details to implement a solution.

In diffuse mode, your mind is more relaxed, and your synapses more readily span different parts of the brain. This is also important for solving problems because it facilitates connecting diverse ideas, allowing you to see the big picture or come up with creative solutions. But it only processes abstract concepts, not detailed information, so to solve most problems, you have to alternate between the two modes.

Your brain will alternate between the two modes naturally, but if you focus intensely on a particular task or problem for a long period of time (like a couple of hours), your brain can get stuck in focused mode, preventing you from progressing beyond a certain point because in focused mode, it can’t see the big picture. This is known as the Einstellung effect.

The Einstellung effect could explain the behavior that Clausewitz observed in his troops: They would go into battle focused on a particular assignment, and the mental stress of carrying out their initial orders under fire would prevent their brains from ever relaxing enough to reassess the big picture. It wasn’t that they’d lost their original focus, but rather that they were unable to shift their focus as the battle progressed, at least not without help from fresh reserves.

As you work on tasks and solve problems at work, at school, or at home, you may not have the luxury of calling in a fresh replacement whenever you fall victim to the Einstellung effect, but the good news is you don’t need to. There are other ways of breaking out of the Einstellung effect so that you can refocus and start making progress again. According to Oakley, all you have to do is deliberately switch your brain into diffuse mode for a while. You can do this by taking a break from the task at hand and going for a walk, taking a nap, or tending to chores that don’t require much thought. As soon as the problem that you were stuck on has been cleared out of your conscious mind, you can re-attack it with a fresh perspective.

Making the Most of Victory by Pursuing the Enemy

As soon as the enemy begins to retreat from the battlefield, Clausewitz urges you to pursue them. Take possession of any cannons or supply wagons that they abandon in their haste to get away, take any stragglers prisoner, and keep the retreating army under fire so they never have a chance to rest and regroup.

Ideally, he says your army should follow B’s retreating force until they try to make camp for the night and then attack their camp, forcing them to retreat again. Then, your army can make camp and get a good night’s rest while B’s army marches through the night, unable to see if you’re still on their heels. If you can catch up with them and attack their camp again the next day, then they'll have to march through the night again, and so on.

(Shortform note: Modern science supports the effectiveness of weaponizing sleep deprivation as Clausewitz advises. As Bill Bryson notes in The Body: A Guide for Occupants, studies have shown that sleep deprivation can be fatal. Lab rats that are prevented from sleeping die within a month, and in humans, genetic disorders that make sleep impossible also lead to death. Thus, in principle, Clausewitz’s tactic of preventing the enemy from sleeping by attacking their camp each night could directly inflict massive casualties if you kept it up long enough.)

Making the Most of Engineering Victories

Clausewitz points out that you can benefit a lot more from winning a battle if you exploit your victory by pursuing the enemy. Similarly, in high-tech business sectors, successfully developing a new product that provides new or substantially improved capabilities can be a major engineering victory. But it won’t do your company’s bottom line any good unless you exploit it by pursuing customers through marketing and effective distribution. And, just as Clausewitz captured enemy supplies, artillery, and soldiers by pursuing his retreating enemy, effective marketing is what allows you to capture market share.

Along these lines, in Perennial Seller, Ryan Holiday emphasizes that you can’t make a product so good that people will automatically want to buy it—you have to show them why it’s worth buying through marketing. He then offers tips on how to market a product effectively, like building relationships with key influencers and giving away free samples of your product to spark interest.

The Turning Point of a War

You continue to chase the remnant of B’s retreating army until they reach a garrisoned fortress and take refuge there. On Clausewitz’s advice, you split your forces, leaving some troops to keep the fortress under siege, while the larger portion of your force continues toward B’s capital.

Splitting your forces is generally not a good idea, because it reduces your strength in numbers. But in this case, it’s warranted because the siege will take time, and the longer it takes you to reach B’s capital, the more time they’ll have to raise additional troops and prepare their defenses: Time is on their side.

Clausewitz explains that invasions such as this often reach a turning point because time is on the side of the defender, while both time and distance are working against the attacker. The longer your army is out in the field, the more your force will get worn down: You lose men in battle, even when you win the battle. You also lose men while marching, as some of them won’t be able to keep up due to illness, injury, or fatigue. Finally, you lose wagons or artillery when they break down in the field or are damaged in battle. And the farther you go into enemy territory, the more troops you have to divert to protect couriers and convoys on the routes that you use to communicate with your home country.

So the longer you continue your attack, the weaker you become while your enemy becomes stronger. As Clausewitz points out, this doesn’t matter if you completely defeat your enemy while you’re strong enough to do so, which is what you’re trying to do in this case.

But if you wait too long, or if you weren’t strong enough to begin with, then you’ll reach a point where the enemy can match your military strength. Ideally, just before you reach that point, you would break off your attack and switch to building up your defenses in the territory you’ve conquered. Then, time begins to work in your favor. But Clausewitz opines that it’s almost impossible to know exactly when you’re going to reach that turning point. He observes that most commanders either halt their attack prematurely or else press the attack until they’re badly overmatched and can’t defend what they conquered.

Turning Time and Distance Against Your Enemy

As he describes how an attacking army becomes weaker the farther it travels, Clausewitz largely takes for granted that this is a disadvantage to the attacker and thus an advantage to the defender. By contrast, when Sun Tzu writes about the same principle, he presents it as an advantage that can go to whichever side makes better use of it.

Sun Tzu advises forcing your opponent to travel as far as possible while you travel as little as possible so that their army wears down faster than yours. He says that, even as the attacker, you can do this by moving a little way toward a vulnerable city, sacred shrine, or other asset that your enemy will then have to move his army to defend. Then, you change course for a different target, and so on, forcing him to march his army all over the place. As long as your enemy is wearing down his army faster than you are wearing down yours, time is on your side.

The Element of Surprise

As you approach Country B’s capital, you finally meet the main body of B’s army. To your surprise, you’re outnumbered by about two to three. As we’ve discussed, there is always some uncertainty in war. Clausewitz points out that other military theorists put a lot of emphasis on gaining an advantage by deceiving the enemy or attacking quickly, in the hopes of catching the enemy by surprise.

He says that in reality, the element of surprise is overrated. The only “surprise” that really makes a difference is finding enemy troops where you didn’t expect to find them, or in greater numbers than you expected, and these kinds of surprises are usually more a matter of chance than deliberate deception.

Is Secrecy Overrated in Business, Too?

As we discussed earlier, Sun Tzu advocates making extensive use of secrecy and deception to gain advantages over the enemy through the element of surprise, unlike Clausewitz, so there is a difference of opinion between military strategists on the relative value of secrecy. There is a similar difference of opinion among business strategists on the value of secrecy and legal protections when it comes to technical information and intellectual property.

Some argue that when you’re developing a new product, it’s important to keep all the details a secret until you can secure patent protection so that someone else doesn’t steal your idea and the profits you would have made on it. They advise that nondisclosure agreements can help ensure that your secrets don’t get out, and provisional patent applications can give you a head start on securing patent protection.

Others argue that patent protection (and the secrecy that goes along with keeping an invention patentable) is seldom worth it because obtaining a patent is a long, expensive process. In some cases, by the time your patent is granted, your invention may already have become obsolete. If you tried to keep it a secret all that time, then you missed out on the opportunity to sell it while it would have been profitable.

Retreat

You meet the enemy in battle, but it soon becomes clear that you’re not going to win, or at least not win a decisive victory that would allow you to achieve your political objective of crushing B’s army and occupying their capital. Clausewitz advises you to execute an orderly retreat while you can still do so with your fighting force intact.

To do this, you assemble a rear guard from your best remaining troops and use them to defend a series of defensive lines. Their orders are to hold each line for a certain amount of time and then fall back to the next line, such that B’s army is continually under fire as they pursue you.

As Clausewitz explains, this provides two major advantages, one physical and the other psychological. The physical advantage is that delaying any pursuit allows your army to retreat slowly enough that you don’t have to abandon supplies, artillery, wounded soldiers, or others who wouldn’t be able to keep up with a fast march.

The psychological advantage is that, as the rear guard succeeds in holding off your pursuers, these little victories help to rebuild your troops’ confidence after it was shattered by losing a battle and having to retreat.

That said, Clausewitz also cautions that after losing a major battle, it’s almost impossible for an army to fully recover its confidence and morale without outside help. Usually, this outside help takes the form of fresh reinforcements: The additional troops increase the physical strength of your army, and, perhaps more importantly, this gives your army tangible reason to believe they’re no longer inferior to the enemy.

In this case, let’s say that you make it back to the fortress that you besieged and rejoin the forces conducting the siege. Meanwhile, B’s army has been weakened by the long chase and the casualties that your rear guard inflicted. Now you’re potentially able to match their strength again.

Dealing With Defeat

Most people will never lose a major military battle because most people aren’t high-ranking military commanders, but everyone experiences failure in some area of life at some point. We can extract a number of universal principles for dealing with failure from Clausewitz’s advice on military retreat, which other authors have applied to dealing with other types of failure.

1. When you fail, admit it. If a military commander refuses to admit that he can’t win a particular battle, he’ll get all his soldiers killed and thereby preclude any future victories. As Clausewitz says, it’s better to retreat while your army is still intact. Similarly, as Ryan Holiday discusses in Ego Is the Enemy, you can waste a lot of resources trying to salvage your failed endeavors when you refuse to admit failure. Refusing to admit failures also prevents you from learning from them, which may doom your future endeavors.

2. Retreat without giving up. Clausewitz insists on making the enemy fight for every inch of ground as they pursue your retreating army, citing both physical and psychological benefits. Similarly, in Make Your Bed, retired Navy SEAL William McRaven insists that when you fail at something, it should motivate you to try again and try harder, not to give up. So even as you retreat from one failure, keep fighting to prepare for your next attempt. What your “fight” looks like depends on the nature of your endeavor. For example, if you’re competing in sports and lose a match, maybe you fight your way out of failure by adopting a more rigorous training routine.

3. Make changes for the better. Clausewitz asserts that an army needs external assistance, such as fresh reinforcements, to recover from a defeat. Similarly, to have any rational hope of succeeding where you failed before, you have to learn from the failure and make changes that will prevent it from recurring. In Think Like a Rocket Scientist, Ozan Varol discusses learning from failures and emphasizes the importance of addressing the real cause of the failure, not just the superficial symptoms.

Stalemate

By this point, you’ve done a lot of marching, crossed a river under fire, defeated some of Country B’s forces, divided your army to lay siege to a fortress, lost a battle against B’s main army, retreated from defeat, and regrouped your main fighting force. However, instead of culminating in a decisive battle between your main army and B’s main army, the war now devolves into a stalemate: It has become apparent that, at present, you don’t have the military strength to achieve your original political objective (crush B’s army completely and occupy their capital). Yet any of B’s territory that you can occupy gives your country more to bargain with in negotiations. Meanwhile, B’s army doesn’t have the strength to crush yours with a decisive counterattack, or at least B’s commander isn’t confident that they do and isn’t willing to risk losing a decisive battle with their main force.

So you both wait for the situation to change. You send out a few raiding parties to ambush B’s supply convoys, and B retaliates in kind. Your raiding parties and convoy escorts fight minor skirmishes with each other, but you both avoid major battles. Clausewitz assures you that this is normal. In fact, situations where both sides are basically just waiting for circumstances to change make up the majority of the time you’ll spend at war.

Timing Your Attack

Sun Tzu identifies waiting for the right time to strike as a key element of strategy in war. If your enemy is strong and ready for a fight, he advises avoiding battle—even if your army is even stronger. Instead, wait until your enemy demonstrates some kind of vulnerability, such as being tired and weak while your army is still strong, and then attack suddenly.

Clausewitz alludes to this element of strategy in his observations about wars dragging on without any decisive battles because both sides are relatively evenly matched and too prudent to risk defeat in a major battle. But he doesn’t discuss the strategy of waiting for the right time to strike to the extent that Sun Tzu does.