Russia famously quit in early 1918 after suffering two revolutions, culminating in a Bolshevik (Communist) takeover. The Bolsheviks tried to keep fighting in the war, but were forced to quit because a civil war began.
After a large Austrian army lost a major battle with Italy in October 1918, their military practically ceased to exist. Disillusioned people across the empire started breaking away, and what was left of Austria-Hungary was forced to surrender.
The Ottoman Empire was largely undone when its Arab territories revolted against them. With British aid, they were successful.
The German Navy mutinied in early November 1918. The mutiny quickly grew until it became a full-blown revolution, and the emperor was forced to flee. A few days later, Germany surrendered, ending the war.
