Like many Americans, I’ve been horrified by the recent Russian invasion of Ukraine, and inspired by the brave resistance of the Ukrainians. The attack also reveals much about the realities of modern warfare. Today I’m going to talk about how the changing demographics of both cities and militaries have aided the Ukrainian defense.

Initially, the Russian attack blitzed through Ukrainian trenches and other conventional defenses, but it stopped short at the suburbs of Ukraine’s cities. Kharkiv, Chernihiv, Mariupol, and Kyiv have turned into modern-day fortresses and stopped the Russian advance, causing Russian soldiers to resort to brutal siege warfare tactics. National defense experts now predict that Russia will have extreme difficulty fighting its way into Ukrainian population centers, despite earlier predictions that Ukraine would fall rapidly.

Defense of Modern and Premodern Cities

Modern cities are one of the most formidable defensive systems in contemporary warfare, arguably more defensible than cities at any other time in history. This is a shocking fact because modern cities are also the first cities to be designed without any considerations toward defense. 

Every premodern city was built with defense as a top priority. All ancient Greek cities had a center (acropolis) built on a fortified hill. Most cities also had impressive external fortifications in the form of multiple layers of walls. They did this despite the fact that defensive measures, almost by definition, make life less convenient. Hills are a beast to climb by yourself, let alone when transporting food, water, or equipment. Walls constantly had to be destroyed and rebuilt in order to accommodate growing populations and urban sprawl. Yet these inconveniences were worth it because premodern cities were exceedingly vulnerable to attack.

Modern cities, on the other hand, are typically laid out with absolutely no consideration for military affairs. On the contrary, cities go to great lengths to be as easy as possible for people to enter and exit with a minimum of inconvenience or blockage. This is the very opposite of what makes an effective defense. 

Why then, have modern cities proved so defensible? Today, I would like to point to two key changes that make today’s cities into accidental fortresses. The first one is that cities have gotten way, way, bigger. The second is that armies have gotten far smaller. Combined, these two trends make conquering cities vastly more difficult than ever before. 

Growing Cities

Let’s start with cities. It’s not exactly a shocker to say that the number and size of cities have grown immensely in recent history. In 1960, only about 33 percent of the world’s population lived in cities. Today, that number is over 55 percent and growing rapidly. The ratio is even more lopsided in Ukraine, where about 70 percent of the population lives in cities (it bears mention, however, that this commonly-cited data does not always apply a universal standard for what counts as an urban area). 

It makes intuitive sense that the larger a city, the harder it is to occupy. There are more buildings, more people who might resist, and a greater land area to occupy. However, one might assume that as urbanization increases, so too would the size of national armies, making the ratio between city population and military size a net neutral.

After all, the higher an urban population, the more centralized a country is, and the easier it is for governments to collect revenue, money which in turn will be used to fund the military. City dwellers are far more efficient to govern and tax than those in rural areas. It is for that reason that CINC scores, one of the main analytical tools used to describe national military power, factor in urban population as a key metric (more on CINC scores here). Urbanization is part of the reason that national governments today take in orders of magnitude more revenue than they did a few centuries ago.

Smaller Militaries

The problem is that, contrary to what you might assume, standing armies haven’t gotten larger. Take the example of Russia. For most of the 1800s, the Russian military had a peacetime strength of roughly 700,000 – 900,000 soldiers, even as its population grew from about 45 million to about 130 million. Post-Soviet Russia has almost the exact same size military, despite an economy that is almost infinitely more prosperous when measured in absolute terms. This is only one example, but it is far from the most extreme. The peacetime 21st-century French military is less than half the size of the peacetime 19th-century French military.

There are three main reasons for the lack of change in the size of armed forces.

Firstly, interstate conflict is less common in the 21st century than it was in the 19th, which disincentives large armies. But that is only part of the answer. As the last few weeks have shown us, Russia is more than happy to launch offensive wars, so its small army seems to have been deemed sufficient for wartime.

The second reason is that many countries, including Russia and France, have seen only limited population growth. No matter how much money you have, it’s not advantageous to draw too high a percentage of your population into the military, so Russia and France are working with a military personnel cap that has changed little over the last few centuries. 

The final reason, and the one I’m going to focus on, is the economic cost of maintaining a military. Military operations have become exponentially more expensive over the last two centuries. Take the United States. In 1850, the U.S. government spent roughly $7,000 (adjusted for inflation) for each man in its armed forces. By 1900, that figure had grown to about $10,000 per man. In 1950, it had skyrocketed to $116,000 per person, and by 2000, it was $360,000! And keep in mind, these are peacetime numbers. Wartime expenditures like the ammunition expended, the equipment damaged, etc, etc, have also skyrocketed. 

Think about it. Back in the premodern era, soldiers had only the most basic equipment. Often, they were expected to provide their own weapons, armor, and food. Today, however, governments pay not only for basics like food, soap, weapons, and health insurance, but the high-tech high-price instruments of modern war.

Getting Ratioed

The net result of all this is simple. Over the past decades and centuries, cities have grown far faster than the overall population, while militaries have decreased in size, often in relation to population, but sometimes even in absolute terms. This means that ever-smaller armies are attacking ever-bigger cities. It doesn’t help that most of the top-shelf tanks, planes, and missiles that modern armies spend so much money on is of little use in urban warfare.

One under-appreciated aspect of historical sieges is that, in many cases, the attacking army actually outnumbered the total population of the besieged city. During the 1453 Siege of Constantinople, for example, the population of Constantinople was a mere 50,000, compared to well over 100,000 Ottoman soldiers outside. The same pattern is true for other sieges as well. Most historians estimate the population of 1st century Jerusalem to have been about 60,000, while the Roman army that attacked it during the Great Jewish Revolt numbered about 70,000 (though the population of Jerusalem during the siege was closer to 120,000 due to refugees from the countryside). At the 1683 Siege of Vienna, 150,000 Ottomans attacked a city of about 90,000 inhabitants.

For comparison, let’s look at the siege of Kyiv. Kyiv has a population of just under three million. The Russian invasion force, on the other hand, numbers less than 200,000 soldiers in the entirety of Ukraine. That means fewer than a third of that total are actually anywhere near Kyiv. One one hand, the enormous ratio of inhabitants to soldiers means that the Russians have resorted to brutal and destructive attacks on civilians, but that measure is an indication of the almost impassable obstacles the Russians face. 

Not only are cities harder to conquer than ever before, but invaders see them as more essential to conquer. As countries wealth becomes increasingly concentrated in the cities they become ever-more strategically vital. This makes offensive warfare far more difficult and complicated. I fear it will also make warfare far more destructive and costly in civilian lives, since population centers will see bitter and protracted fighting.

Certainly, there are plenty of other reasons the Russians have had difficulty attacking the Ukrainian cities, from poor planning and low morale to the weaponry and tactics being employed on the ground. Yet long-term demographic forces also play a crucial role in the heroic resistance of the Ukrainians.