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From Wikipedia, here are some significant dates in atomic bomb development:
1933 - Leó Szilárd realizes the concept of the nuclear chain reaction, although no such reaction was known. He invented the idea of an atomic bomb in 1933 while crossing a London street. He patented it in 1934. (British patent 630,726)
1939 - October - US President Franklin Delano Roosevelt receives the Einstein–Szilárd letter and authorizes the creation of the Advisory Committee on Uranium. The Uranium Committee has its first meeting on October 21, and $6,000 was budgeted for conducting neutron experiments.
1940 - April - MAUD Committee (Military Application of Uranium Detonation) established by Henry Tizard to investigate feasibility of an atomic bomb.
1941 - October - President Roosevelt receives MAUD report on the design and costs to develop a nuclear weapon. Roosevelt approves project to confirm MAUD's finding
OK, everything gets moved up by 6 years and the first A-Bomb test is in August 1939 in New Mexico. World War 2 gets postponed until Germany has its own nukes. What happens then? What do the UK, France, USSR, Japan and Italy do?