When you spend a little time on this subreddit you'll quickly learn that spears were the dominant weapon of the pre-gunpowder world. However, I have also seen multiple times that the Roman army, in at least one of its incarnations, used swords as its primary weapon and throwing javelins non-melee purposes.

I recognized that the Roman army varied significantly across history, so is it the case that this was a relatively short-lived phenomenon, or are there other reasons to explain this seeming exception to the rule?