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Do wars cause empires and societies to collapse? History’s answers are never black and white.

Warfare is frequently blamed for the collapse of empires. But histor­ical examples of societies that fell after warfare can be placed in two categories. In one, societies that were strong and thriving and showed no signs of being on the verge of collapse become embroiled in a conflict with an adversary of superior manpower or armaments. Had the attack or war not occurred, the defeated society probably would have continued to thrive for the foreseeable future. In these instances, war or aggression clearly led to the demise of empires. Prime examples are the Persians, defeated by the forces and strategies of Alexander the Great in 330 BC; Carthage and its final fall to Rome in 146 BC, and the Aztecs, who could not survive the onslaught of the conquistadors under Hernán Cortés in the early 16th century.

In the other category are societies that held foreign aggressors at bay for many centuries while dealing with their own internal problems. Finally, those problems weakened them to the extent that they could no longer fend of the enemy. The fall of the Khmer Empire, centered in present-­day Cambodia, is a classic example. Coalescing in the early ninth century, it came to dominate the region. Impressive builders, the Khmer constructed monumental temple complexes in their capital, Angkor, at the time the largest city in the world. They also created an elaborate water system of reservoirs and canals. Vital to agricultural production, the system was difficult to maintain: Silting and other operational problems, exacerbated by the increased variability of the Asian monsoons—unprecedented wet years interspersed with unprecedented dry ones—began to take a toll. Meanwhile, Thai interlopers, driven themselves by invaders from the north, began to encroach on Khmer territory. By the early 15th century, climatic pressures, a failing water system, and warfare with the Tai had led to the decline of the Khmer Empire.

The Western Roman Empire suffered a similar fate, though scholars continue to debate the “real” cause of its demise. Clearly, by the third century ad, internal and external problems had begun to create fissures in the great Roman edifice. The empire suffered from a debased currency, chronic successional and environmental problems, and geographic limitations—it could no longer fill its coffers by conquering rich neighbors. And then the “barbarians”—Germanic tribes in the north and Huns in the east—who had been at Rome’s frontiers for many centuries, pressed in. Some historians believe Rome fell because those barbarian forces had become more numerous or better organized or armed than a Rome of unchanged strength; other scholars feel that Rome’s growing internal problems had left it weakened and no longer able to defend itself against the barbarian wave.

History is never black and white, and the gray continuum of internal and external pressures generally applies to the collapse of societies. Simply put, when a society’s internal problems weaken it, hostile neighbors see an opportunity for conquest.


Geographer Jared Diamond is the Pulitzer Prize–winning author of Guns, Germs, and Steel (1997) and Collapse (2005).

Originally published in the April 2014 issue of Military History Quarterly. To subscribe, click here.