Winston Churchill -- he was the main reason WWII wasn't won by Germany. When all of Europe was overrun, America still held herself aloof, and England herself was being bombed daily, he refused to surrender. There has been a ton of material written about him, so you should have no trouble find research sources for more info.
FDR -- he brought America into the war. I seem to remember reading somewhere that he had provided covert support to Britain for a while, and was actually eager to join the fight against Hitler, even though he campaigned in favor of keeping American out of the war. He just couldn't summon up enough public support for involving America until Pearl Harbor. It's been awhile since I studied this, though, so I could be mistaken.
Good luck
Sean, EssayForum.com