Which caused the US Army to take over the atomic research project being conducted at Columbia University in the early 1940s?

The United States entered World War II after the attack on Pearl Harbor in December 1941. The government then moved to organize and accelerate atomic research, and the Army took control of the project at Columbia University. Einstein’s letter helped alert President Roosevelt to the possibility of an atomic bomb, but it was not the immediate cause of the Army takeover.

AI-written and checked against the verified answer.