WW1 was the first war that was truly global, for the war that started in Europe also affected nation's in the Asias as well as the Americas. With the subcontinent and a whole different continent combined, and with most of the world's country in the war, it can be described as the world's first world war.
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The primary military advantage the Romans had was a massive army, the greatest in overall size. Although the greatest disadvantage is that other armies such as Sparta were trained far more heavily.
