You don’t need to be the biggest ancient history buff to know that the Roman army was among the greatest military forces the world has even seen. When it came to conquering territory, they were remarkably efficient. The empire swelled to more than five million square kilometers in size, before ultimately collapsing because its sheer size made it utterly impractical to defend and administer. Along the way, the Romans conquered Greece, Gaul, and so many other regions in between. Needless to say, they fought numerous bloody wars to claim and maintain control over such lands. As successful as they tended...