5 of the Most Formidable Armies Throughout History

The Achaemenid Empire was founded by Cyrus II when he overthrew his grandfather Astyages of Media to take the throne circa 550 BCE. The army he established was extensively trained, highly organized, and incredibly powerful. Would-be warriors began a five year program of training at age 15, covering diverse categories including horsemanship, hunting, swimming, archery, martial arts, and agriculture. Achmaenid soldiers were expected to be self-sufficient in battle, reducing the need for complicated supply chains. When they entered the service, soldiers were organized using the decimal system: 10 men to a company, 10 companies to a battalion, 10 battalions to a division, and 10 divisions to a corps. The empire also had a dedicated war council, which would draw up plans of battle prior to any engagement. Cyrus quickly conquered Lydia, Elam, and Babylon in succession, and with each gained legions of new warriors for his army.

Darius I, who took over in 522 BCE, also raised one of the most powerful naval forces of the ancient world. It included many triremes, enormous warships with a crew of 200 men each. These ships provided Darius’ army a unique advantage in the empire’s 492 BCE invasion of Greece, which raged for two years until the city-state of Athens pulled off a shocking victory at Marathon. This sparked a punitive expedition in 480 BCE under Xerxes I, which was only repelled by the sacrifice of the Spartan army in the mountain pass of Thermopylae.

Under the direction of bold and fearless leaders, and through strategies of ruthlessly efficient conquest, the Mongol army became one of the strongest in history. Its most revered leader, Genghis Khan, was born circa 1162, a descendent of the warlord Bodonchar Munkhag. By the age of 15, he had some 20,000 warriors under his command. He led them in consolidating his power, uniting the Mongols into a military powerhouse. While often remembered as ferocious warriors, the Mongol army was also tactically advanced. Mongol soldiers were trained in archery, horsemanship, and unit tactics, and held to a standard of strict discipline. Their infantry units were composed of horse archers, whose light equipment allowed them to make quick maneuvers, and heavy, impenetrable cavalry. 

They approached many battles by first firing arrows to scatter the enemy forces, then riding the heavy cavalry through to clear out the stragglers. They also frequently employed tactics of flanking or luring the enemy into disadvantageous positions. Mongol soldiers often took several horses with them at once, and would switch between them as the animals tired out. Their high mobility allowed them to travel up to 100 miles in a day, and quickly relay messages back and forth from the front line. These tactics allowed the Mongols to assemble the largest contiguous land empire in history by the 14th century, ranging from eastern Europe to the sea of Japan. 

The Ottoman Janissaries were among history’s first examples of a disciplined unit of elite soldiers. As a whole, the Ottoman army developed, alongside the empire, from a nomadic cavalry into a modern combat force between the 14th and 19th centuries. The Janissaries, once a unit of converted Christians under forced conscription, became Europe’s first permanent army of professional soldiers. They underwent extensive training—first in archery, and then in firearms as soon as the 1440s. Janissaries were accompanied in battle by messenger and engineering units to fortify their defenses and smooth communication. The Ottoman army’s investment in gunpowder, a novel technology at the time, made them a force to be reckoned with. 

The Janissaries proved themselves in the 1453 fall of Constantinople, a battle that raged over 53 days. Their forces highly outnumbered the Byzantine army within the city. Under the command of Sultan Mehmed II, the Ottoman army and navy attacked concurrently, surrounding the Byzantine capital on all sides. They overcame the seemingly impenetrable fortifications of the city, smashing through its walls with powerful cannons. Thanks to the might of their gunpowder, strength of their numbers, and competence of their communication, the Ottoman army managed to conquer the ancient city for only the second time in its history. Their victory sent shockwaves of fear through the Christian world, and established the Ottoman Empire as a dominant global power. 

Following the collapse of the feudal government in 1467, Japan’s government plunged into the chaotic Sengoku period. Rival warlords vied for power with varying degrees of success, and one of the most successful was Oda Nobunaga. Born June 23, 1534, he was the second son of Oda Nobuhide, a powerful clan leader, and took over that position at the age of 17. Oda’s army often found strength in numbers, but even in those early days when his contingent was small, their head-on, aggressive tactics won them many unlikely victories. When his father’s rival Imagawa Yoshimoto overtook Marune Fortress in 1560, Oda launched an offensive, catching the Imagawa warriors with their armor off in celebration. Oda’s army captured and executed Imagawa, causing the clan to surrender and effectively wiping his rival off the map.

Oda then forged an alliance with the Matsudaira clan against the Saitō clan. He was responsible for the coup that put Ashikaga Yoshiaki on the shōgun’s throne, and fought to maintain Ashikaga’s rule against a series of challengers. Ashikaga would later turn on him, after which Oda’s army forced Ashikagi into exile. His successor was Oda’s Matsudaira ally Tokugawa Ieyasu. Oda also had his hands full with the Ikkō-ikki uprising, a Buddhist-aligned rebellion against the military rule of the daimyō. Still, he rose to the top by 1582, when, with an enormous army, most of the mainland under his control, and the shogun firmly in his corner, he became the most powerful man in Japan. However, his reign was cut short during an attempted coup by Akechi Mitsuhide, a former bodyguard of Ashikaga. To date, Oda and Tokugawa are remembered among the great unifiers of Japan.

Although most known for its size and effectiveness in repelling Nazi Germany’s invasion during World War II, the Workers’ and Peasants’ Red Army was first established in 1918, under the leadership of Vladimir Lenin. By 1941, however, the Red Army was woefully unprepared for the oncoming invasion. They lost millions of men in the first few weeks of what they called the Great Patriotic War. However, the Red Army was quick to get its act together. Red Army soldiers were ordered never to surrender—dying in battle was thought preferable. This was partially supported by a campaign of propaganda that compared Hitler’s invasion to Napoleon’s a century earlier. Joseph Stalin funneled massive amounts of money and manpower into the counterintelligence wing of the army, eradicating the threat of foreign spies. Stalin also launched a campaign of mechanization, investing millions in the construction of tanks, fighter jets, and motorized guns. 

In 1942, Nazi Germany attempted to seize strategically vital oil fields near the city of Stalingrad. Stalin sent all available Red Army troops to defend the southwestern city; local civilians aided the effort by constructing trenches and other fortifications. The Soviet forces remained as close to the German frontline as possible in an effort to slow their progress toward the city. The Battle of Stalingrad took place over five months of bitter, pitched fighting. Positions frequently changed hands, and there were heavy casualties on both sides—nearly a million Soviet troops died in combat at Stalingrad alone. However, their sacrifice was not in vain, as the invading Nazis were eventually pushed back. Their crushing defeat was one of the most significant of the war, representing a tidal shift toward an eventual Allied victory.