What was the most important event propelling the United States into the Grand Alliance with Great Britain and the Soviet Union during WWII?

Question

A.
the discovery of Nazi concentration camps
B.
the America First policy
C.
Germany's invasion of the Soviet Union
D.
the Japanese attack on Pearl Harbor

Answer 1

D. the Japanese attack on Pearl Harbor

The Japanese attack on Pearl Harbor on December 7, 1941, was the most important event that propelled the United States into the Grand Alliance with Great Britain and the Soviet Union during WWII. The attack led to the United States officially entering the war and committing to fighting alongside its allies against the Axis powers of Germany, Italy, and Japan.