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. Wartime urgency led the government to place the atomic research effort under Army management, beginning the Manhattan Project. Albert Einstein’s letter helped alert officials to the possibility of atomic weapons, but the U.S. entry into the war prompted the Army takeover.

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