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Einstein's 1939 letter helped initiate the U.S. effort to build an atomic bomb, but work proceeded slowly at first. Two other findings in 1940 and 1941 demonstrated conclusively that the bomb was feasible and made building the bomb a top priority for the United States: the determination of the "critical mass" of uranium needed and the confirmation that plutonium could undergo fission and be used in a bomb. In December 1941, the government launched the Manhattan Project, the scientific and military undertaking to develop the bomb.

Einstein wrote to U.S. President Franklin Roosevelt to warn him that the Nazis were working on a new and powerful weapon: an atomic bomb. Fellow physicist Leo Szilard urged Einstein to send the letter and helped him draft it.

July 1940, the U.S. Army Intelligence office denied Einstein the security clearance needed to work on the Manhattan Project. The hundreds of scientists on the project were forbidden from consulting with Einstein, because the left-leaning political activist was deemed a potential security risk.

“avoid a charge of ‘attack without warning,’

U.S. forces had not received warning by the time the first wave of Japanese planes attacked Pearl Harbor soon before 8 a.m. A second wave followed an hour later.

The American Response

In 1949, the prospect of further Communist expansion prompted the United States and 11 other Western nations to form the North Atlantic Treaty Organization (NATO

The Soviet Union and its affiliated Communist nations in Eastern Europe founded a rival alliance, the Warsaw Pact, in 1955.

Conflict between the Western nations (including the United States, Great Britain, France and other countries) and the Communist Eastern bloc (led by the Union of Soviet Socialists Republics or USSR) began almost as soon as the guns fell silent at the end of World War II (1939-45)

On this day in 1945, President Franklin Delano Roosevelt passes away after four momentous terms in office, leaving Vice President Harry S. Truman in charge of a country still fighting the Second World War and in possession of a weapon of unprecedented and terrifying power.

it was about 1 p.m. that the president suddenly complained of a terrific pain in the back of my head and collapsed unconscious. One of the women summoned a doctor, who immediately recognized the symptoms of a massive cerebral hemorrhage and gave the president a shot of adrenaline into the heart in a vain attempt to revive him.

Eleanor delivered her speech that afternoon and was listening to a piano performance when she was summoned back to the White House. In her memoirs, she recalled that ride to the White House as one of dread, as she knew in her heart that her husband had died.

Born on Feb. 13, 1923, and raised in the hills of West Virginia near the town of Myra, Charles “Chuck” Yeager entered military service as soon as he could, joining 17 classmates who enlisted after high school graduation to fight in World War II.

Breaking the Sound Barrier

Assigned to a host of test flights, Yeager was soon selected as pilot during the Air Force’s attempt to break the sound barrier; he would fly a super-sonic plane called the X-1, nicknamed Glamorous Glennis in honor of his wife. Rocket-like and so tight and compact that it had to be dropped from a cargo plane to conserve fuel, the X-1 was the United States’s chance to top Mach 1.

On April 30, 1945, Adolf Hitler shot himself in the head in his bunker beneath Berlin, moments after Eva Braun, his wife of just one day, killed herself by ingesting cyanide.

Adolf Hitler was aware that he would not win the war he had started.

“He added that he would never allow himself to be taken prisoner by the Russians but intended to shoot himself.”

North Atlantic Treaty Organization was founded in response to the threat posed by the Soviet Union

Alliance’s creation was part of a broader effort to serve three purposes:

deterring Soviet expansionism, forbidding the revival of nationalist militarism in Europe through a strong North American presence on the continent, and encouraging European political integration.