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Revealed: The 6 Most Lethal Armies in All of History

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.

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