Roosevelt
The most powerful leader in Russia during World War I was Nicholas II. In addition, Nicholas Nikolaevich and Aleksei Brusilov were leaders during WW1.
Emperor HiroHito
President truman
Winston Churchill .
FDR
FDR
FDR served as POTUS during WWII.
Franklin D. Roosevelt
Joseph Stalin was the leader of the Soviet Union during World War II.
Admiral Chester Nimitz (for most of the war)
Hideki Tojo was the leader of japan in world war 2.
There was no single leader during WW1. Each country had their own leader.