Why it took such a long time for an army of physicists to realize the atomic bomb, while in theory it seems pretty easy? I think most of us know about the construction of the first atomic bomb at Los Alamos, with 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 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.
 A: AS you say, by the time the Manhattan project began in earnest, the fundamental principles were extremely clear. The length of development time needed was owing to predominantly technical hurdles, not predominantly lack of physics knowledge: 


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*A relatively minor problem was the lack of computing power, which hindered the development of models that would foretell critical masses and also the hydrodynamics of the explosion itself. The latter governs how swiftly the potential bomb breaks itself apart thus quenching the reaction. This problem was overcome simply by the development of simple analytical models for symmetrical systems that did not deviate in shape from the final bomb too much;

*The first major problem was the purification of uranium to enrich it enough to allow a chain reaction - only $^{235}{\rm U}$ is fissile. There were no current technologies that could separate isotopes on the scale required and these had to be developed pretty much from scratch;

*The second major problem, once enough plutonium came on hand (plutonium is relatively easy to separate chemically from spent uranium fuel) was the presence of $^{241}{\rm Pu}$. There was no workable technology for removing this "contaminant". $^{241}{\rm Pu}$ is highly radioactive and, if present in a gun type weapon slug (as it inevitably is), its decay products will trigger the nuclear chain reaction far too early as the subcritical masses are thrust together, unless this happens with stupendously high speed. The potential bomb thus blows itself swiftly asunder quenching the reaction. At first the "tall man" design of bomb was tried, which the researchers thought was long enough to accelerate the subcritical slug enough to assemble the critical mass quickly enough. However, this proved impracticable, at least for a bomb that might be delivered by an aircraft. The only alternative that could assemble the critical mass fast enough was the implosion idea, but to make this work the plutonium has to be compressed extremely uniformly, otherwise the crushed core bulges out sideways and criticality will not be reached. There was a huge amount of technology involved in using explosives to yield perfectly spherical shock waves; this had never before been done.

*All of the above technologies required radical new measurement technologies to be developed so that experimental data could be gleaned. New high speed photographic techniques had to be developed (see the Rapatronic technology, for example), and near GHz bandwidth measurement electronics. Neither of these things existed before the project. 
One could compare this situation with something like the quest for a sustained fusion reaction today, and the huge technological problems it poses. The fundamental physics is perfectly clear and the hurdles are wholly technological.
A: Keep in mind that back then this was all completely new territory for them. They were working with materials that they knew could have killed them all if they got it wrong. They didn't have the benefit of hindsight that we have now.
And it may seem like it's just smashing two masses together but they were making an incredibly complex bomb, meaning it needed to be carefully planned and built with materials that didn't exist before. You don't want the thing going off while you're trying to load it on the plane or in mid flight. With that logic we could say that making a tower is just stacking material on top of each other but how much you plan and design it determines if it's just a tall pile of metal and concrete that could fall down at any time or a reliable sky scraper that people live and work in.
