I think most of us know about the construction of the first atomic bomb at [Los Alamos](https://nl.wikipedia.org/wiki/Los_Alamos_National_Laboratory), with [Robert Oppenheimer](https://en.wikipedia.org/wiki/J._Robert_Oppenheimer) (who said he became "The destroyer of worlds", which goes to show he regretted his participation; nevertheless he **did** participate) in charge of an enormous complex where many, many brilliant physicists (including Feynman) were offered (ordered for participating in?) a well-paid job, housing, food and drink, etc. The project was initiated by Einstein after sending a letter to Roosevelt (correct me if I'm wrong), which seems to contradict his pacifist attitude. But that's not that relevant to my question. Which is:

Why it took such a long time (2-3 years) for all these men (and some women), working on that enormous complex, to construct an actual working device [the first test (called the [Trinity](https://en.wikipedia.org/wiki/Trinity_(nuclear_test)) nuclear test) hit the jackpot], while in principle you "just" have to smash together two masses of Plutonium below the critical mass, which after the smash have a mass above that mass? That **was** known (i.e. in theory) at the time. Was it because it was the beginning of the atomic era, and there was still much to learn? Was it to prevent failure? I've read many times the Nazis were on the verge of constructing one too, and I suppose the Americans knew that too. So why not hurry a bit more? "Luckily", the Americans were first, though there were two dropped on Japan since Germany had already surrendered. Even a third was planned to be thrown because there could be used three different elements in the bomb, and the Americans wanted to see how all three exploded. The second one, dropped on Nagasaki, was i.m.o. totally superfluous.