Babylon, Rome, and the Aztecs had a great deal in common.

Apart from being great empires, what else did they have in common?

Later, around the Renaissance, crossbows, pikes, and guns unseated the knight from military dominance; and the system that best supported that sort of force turned out to be the republic.

As late as 1914, most countries in the Old World were still monarchies. The republics that happened to exist during the Renaissance (Genoa, Venice) were mainly maritime powers, so no crossbows and pikes.

Later, the Industrial Revolution kicked this sort of thing into high gear, with new military paradigms arising every few decades: ironclads, tanks, planes, nukes. Supporting all of these required the economy to be cranked up to the maximum degree possible to make all that stuff, and scientific research as well to figure out the next trick. As it happens, the form of society that seems to work best at that is something resembling a liberal democracy.

The Nazis and Soviets were quite good at military technologies. At least the latter collapsed for reasons unrelated to having weaker army than their competitors.