The Manhattan Project was the code name for a joint project undertaken by the United States, Great Britain and Canada during World War II, the objective being to build an atomic weapon before Germany. While various laboratories had been independently studying nuclear physics for a number of years, the Manhattan Project would be on an entirely different scale; hundreds of the allies' top scientists and engineers were, along with their families, taken to various secret locations around the country where they would stay until the project succeeded.
Working conditions were extremely primitive and security was incredibly tight. The United States Army was in overall charge of the Manhattan Project, while the scientists remained in control over the actual research. The scope of the Manhattan Project was immense, and it involved solving some of the most difficult scientific and engineering projects of the day. The men and women, a collection of soldiers and scientists, endured harsh conditions and physical and mental exhaustion, which were compounded by security so tight that many never saw the outside of the base on which they were stationed. They were sustained by their patriotism and by fear of what would happen to them, their families, and their countries if Germany created the bomb first.
Eventually, of course, the effort paid off. By the middle of 1945 the US had two working atomic bombs. Germany, the original target, had already surrendered, but Japan was still in the fight. The two weapons were dropped on the Japanese cities of Hiroshima and Nagasaki early in August, and the Japanese Empire surrendered shortly thereafter.
From our perspective some decades later, it isn't clear whether the Manhattan Project helped to save the world for democracy or brought it closer to fiery destruction. But the men and women involved believed they were fighting to stop a terrible dictatorship from gaining a new weapon and destroying everything they loved, and they were willing to sacrifice everything to succeed.