Question

An event leading to German surrender was: the uniting of Austria-Hungary the United States joining the Allies the Allies retreating from the Germans the surrender of Russia

Answer 1

An event leading to the German surrender was the fact that the United States joined the side of the Allies - with this happening, there was no way how Germany would be able to win the war anymore. 

Answer 2

I believe the answer is: United States joined the side of the Allies

Before united states joined the Allies, the main military of the Allies consist of troops from Britain, France, and Soviet Union.

After united states join, Germany suddenly have to face an additional of 2 million soldiers. This make the Germany was forced to surrender from the world war I