The two men had a mutual goal: Stop the movement of the Nazis throughout Europe. Since Britain was basically "down for the count", the U.S. had to step up and save them. FDR felt that since he served in WW1 that partaking in WW2 was inevitable. The two met in secret and drew up a plan to possibly take down the Japanese (it was called the Manhattan Project) and kept it secret even from the Russians. During their time together strategizing, they became good friends and when FDR died in April 1945 (a month before he died) Churchill was deeply saddened by it.
Hope this helps!
