I like how you ignore Russia's, um, internal problems at the time :-/

But those internal problems were largely caused by the hardships and stresses of losing the war. Similarly, the proximate cause of the German surrender in WW1 was "internal problems" in Germany (the revolutions in Kiel and Bavaria, etc) but those are inseparable from the fact that the hardships of the war and the psychological sense that Germany was losing put an intolerable strain on German morale. Loss of morale leading to institutional overthrow is often the mechanism by which countries collapse when at war.