Technology

Except the USA too makes small drones. It's asymmetric, shields are temporarily useless, as if your adversary were no longer shooting arrows or soft lead musket rounds, using rifles with fast piercing rounds instead (I know there were shields against rifle rounds in WWI, just didn't work well enough, because those were too heavy).

Sometimes empires crash because of such tendencies, but we are talking about the empire which is propelling them, so - no. They'll just start clearing trade routes with their own cheap autonomous weapons, probably.

Might be genocide involved. Or perhaps with modern precision that won't even be required.