We only have WW2 etc to go on but the thinking is a democray is usually not as well prepared for industrial scale warfare as say fascism or communism in early stages. The problem for autocrats is people who dissent are ignored or worse so you end up with no one saying "Mein Furher an invasion of the Soviet Union will fail" everyone ends up a yesman.
Also intelligence agencies of autocratic regimes are better at internal security than at disrupting the enemy a crucial part of warfare today, also members of autocratic governments waste resources watching each other to ensure there quids in with the leader and as a result not properly watching there enemy.
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