The armies of the 20th Centuary were democratic. Is this a long term trend or a blip?

New Model Army: run by a dictator, but efficient
Napoleon: An autocratic Emperor, but won battles
Roman Empire: worked under both a republic and an Emperor; Byzantine empire was there for hundreds more years
Japanese armies: under a Monarchy. Yes, they lost WW2, but they won most things before that and I don't think being a democracy suddenly makes one beat America.
Alexander the Great, Genghis Khan, Peter the Great...
Vietnamese: Communist
Cambodian: Communist
Red Army (post revolution): Communist

So, loads of autocratic regimes have been whipping all foes before them for hundreds of years.