The Roman legion was an infantry force, and the Romans knew it. This was not a limitation they were ignorant of — it was a structural fact of their military system that they compensated for through a combination of allied cavalry, auxiliary units recruited from peoples with native equestrian traditions, and tactical deployment that minimized the situations where cavalry superiority could prove decisive. The compensation worked well enough that Rome built an empire with an army whose core fighting unit was not the arm — cavalry — that dominated most of the ancient world’s military thinking. Understanding why Rome succeeded despite this, and where it failed because of it, is understanding something important about Roman military power.