Historical Context
In the beginning of World War II in 1939, it was believed that Germany was working on a nuclear weapon. The first Uranium-235 atom had been split just a few years prior along with the discovery of a chain reaction. As the war in Europe worsened, many brilliant scientists and physicists fled to America. Just a day before Pearl Harbor, The decision to make The Manhattan Project a reality happened.
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Model of how the Uranium-235 atom was split, Source: Atomic Archive
In 1938, two German scientists, Otto Hahn and Fritz Strassman successfully split a Uranum-235 atom. After the news of their discovery spread, fears of Germany building a nuclear weapon grew, therefore leading Hahn and Strassman, along with many other European scientists to flee to America. They warned Roosevelt of Germany's research which led to his decision of starting nuclear research.
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Pearl Harbor on December 7th, 1941, Source: History.com.
On December 7th, 1941 Pearl Harbor was surpise attacked by Japanese bombers early in the morning. This event led to America declaring war on Japan and finally entering the war. At this point, creating a nuclear weapon was more serious and necessary than ever.