In an anarchical system like international relations, military power is the ultimate form of currency. A state may have all the culture, art, philosophy, and glitter and glory in the world, but it’s all for naught if the country doesn’t have a powerful military to defend itself. Mao Zedong put it bluntly when he stated: “power grows out of the barrel of a gun.”

Of all the types of military power, armies are arguably the most important for the simple fact that people live on land, and are likely to continue doing so in the future. As the famous political scientist John J. Mearsheimer has noted: “Armies, along with their supporting air and naval forces, are the paramount form of military power in the modern world.”

In fact, according to Mearsheimer, the Pacific War against Japan was the “only great-power war in modern history in which land power alone was not principally responsible for determining the outcome, and in which one of the coercive instruments— airpower or sea power—played more than an auxiliary power.” Nevertheless, Mearsheimer maintains, “land power [still] played a critical role in defeating Japan.”

Thus, armies are the most important factor in assessing the relative power of a nation. But how do we judge which armies were the most powerful in their time? By their ability to win battles decisively and consistently and the extent to which they allowed their countries to dominate other states—a function of land power, as only armies could achieve this type of control and conquest. Here are some of the most powerful armies in history.

The Roman Army

The Roman Army famously conquered the Western world over a period of a few hundred years. The Roman Army’s advantage was tenacity, its ability to come back and fight again and again even in the face of utter defeat. The Romans displayed this during the Punic Wars when despite a lack of knowledge and resources, they were able to defeat the Carthaginians first by waiting them out and then by using the tactics of surprise (by landing an army at Carthage itself).  

The Roman Army gave its soldiers many initiatives to fight for the army with vigor and determination. For poor soldiers, victory in war meant grants of land. For landholders, it meant protecting what they held dear and also gaining additional riches. For the Roman state as a whole, victory meant securing Rome’s security.

All of these initiatives spurred Roman soldiers to fight harder, and morale is a very important ingredient in the performance of armies. Just as important in this was its use of multi-line formations which, among its many advantages, helped the Roman Army replenish front-line troops during battle, where fresh Roman soldiers would square off against exhausted enemies. The Roman Army, often led by brilliant generals, also used mobility to generate offensive advantages, especially against their often defensive-minded enemies.

As a result, in a span of about three hundred years, Rome expanded from a regional Italian power to the master of the entire Mediterranean Sea and the lands surrounding it. The Roman Legions—divisions of the Roman Army which contained professional soldiers who served for 25 years—were well trained and well-armed with iron and were placed all over the empire in strategic locations, both holding the empire together and its enemies at bay. The Roman Army, despite some setbacks, really had no competitors of equal strength anywhere in its neighborhood.

The Mongol Army

The Mongols, who numbered at most one million men when they started their conquests in 1206, managed to conquer and subjugate most of Eurasia in a hundred years, defeating armies and nations that had tens or even hundreds of times the manpower of the Mongols. The Mongols were basically an unstoppable force that emerged seemingly out of nowhere to dominate the Middle East, China, and Russia.

Mongol success boiled down to the many strategies and tactics employed by Genghis Khan, who founded the Mongol Empire. Most important was the mobility of the Mongols and their endurance. To begin with, the nomadic Mongol way of life enabled them to move large armies across amazing distances in short times, as the Mongols could live off of their herds or the blood of their horses.

Indeed, the Mongols’ mobility was enhanced by their heavy reliance on horses. Mongol Cavalrymen each maintained three or four horses to keep them all fresh. Cavalrymen, who had bows they could shoot while riding, gave Mongols distinct advantages over the infantry during the fight. The mobility generated by the horses, as while as their strict discipline, also allowed the Mongols to utilize innovative tactics including hit and run attacks and a primitive form of blitzkrieg.

The Mongols also relied heavily on terror, deliberately inflicting major damages and casualties on their defeated enemies to break the morale of future ones.

Ottoman Army

The Ottoman Army conquered most of the Middle East, the Balkans, and North Africa in its heyday. It almost always overwhelmed its Christian and Muslim neighbors. It conquered one of the most impenetrable cities in the world—Constantinople—in 1453. For five hundred years, it was essentially the only player in a region that was previously comprised of dozens of states and until the 19th century, managed to hold its own against all its neighbors. How did the Ottoman Army do this?

The Ottoman Army began to make good use of cannons and muskets before its enemies, many of whom still fought with medieval weapons. This gave it a decisive advantage when it was a young empire. Cannon took Constantinople and defeated the Persians and Mamluks of Egypt. One of the major advantages of the Ottomans was the use of special, elite infantry units called Janissaries. Janissaries were trained from youth to be soldiers and were thus highly loyal and effective on the battlefield.

Nazi German Army

After the prolonged stalemates of World War I, Nazi Germany’s Army—the Wehrmacht— shocked Europe and the world by overrunning most of Central and Western Europe in a matter of months. At one point, the Nazi German forces even seemed poised to conquer the massive Soviet Union.

The German Army was able to accomplish these enormous feats through its use of the innovative Blitzkrieg concept, which, utilizing new technologies in weaponry and communication, combined speed, surprise and concentration of forces for appalling efficiency. Specifically, armored and mechanized infantry units aided by close-range air support were able to punch through enemy lines and encircle opposing forces. In the opening stanzas of World War II, the aforementioned opposing forces were often so shocked and overwhelmed that they put up only minimal resistance.

Executing the Blitzkrieg attacks required highly-trained, capable forces, which Berlin had in spades.  As the historian Andrew Roberts has observed, “soldier for soldier the German fighting man and his generals outperformed Britons, Americans and Russians both offensively and defensively by a significant factor virtually throughout the Second World War.”

Although Nazi ideology and a melomaniac leader hindered the Wehrmacht’s war efforts, it was ultimately insufficient resources and manpower that brought Nazi Germany down.

The Soviet Army

The Soviet Army (known as the Red Army before 1946), more so than any other army, was responsible for turning the tide of World War II. Indeed, the battle of Stalingrad, which ended with the surrender of the entire German 6th army, is nearly universally cited as the major turning point of the European theatre in World War II.

The Soviet Union’s victory in the war, and its ability to threaten the rest of Europe for the next four decades after the fighting stopped, had little to do with superior technology (outside of nuclear weapons) or military genius (indeed, Stalin's military leadership was absolutely disastrous, particularly early on in World War II, and he had purged many of the more capable commander in the years leading up to it).

Rather, the Soviet Army was a military juggernaut thanks almost entirely to its enormous size, measured in terms of landmass, population and industrial resources. As Richard Evans, the preeminent historian of Nazi Germany, explained: “According to the Soviet Union’s own estimates, the Red Army’s losses in the war totaled more than 11 million troops, over 100,000 aircraft, more than 300,000 artillery pieces, and nearly 100,000 tanks and self-propelled guns. Other authorities have put the losses of military personnel far higher, as high indeed as 26 million.”

To be sure, there were moments of military genius, mainly when Stalin empowered his few capable commanders, and promising technology, notably the T-34 tank. Still, these were not the decisive factors in the Soviet Union’s ultimate success, as its enormous sacrifices continued through the Battle of Berlin.

With the exception of nuclear weapons, Soviet army of the Cold War was not much different relative to its adversaries. While NATO held much of the technological advantages during the four-decade struggle, the Soviet Union enjoyed enormous numerical advantages in many categories, most notably manpower. As a result, in the event of a conflict in Europe, the United States and NATO planned to turn to nuclear weapons early.