While watching a movie, someone suggested that Heisenberg didn't want to succeed in developing a nuclear weapon for the Germans. Was American success due to the will of the people involved? The resources allocated? Sheer luck?
The deck was stacked to the advantage of the Manhattan Project in multiple ways.
First off, America had vaster industrial and natural resources to draw from and didn't have its homefront threatened significantly by invasion or bombing (unlike Germany, Japan, or the USSR).
Secondly, America (and the UK) had been the beneficiary of the vast brain drain of European scientists fleeing the rise of fascism and anti-semitism. The Manhattan Project had an enormous embarrassment of riches in truly world class scientists. To run a nuclear program you typically need at least one truly excellent scientist and enough reasonably competent scientists and engineers to get the work done. But today a lot of the problems in nuclear technology are well understood, the Manhattan Project was doing original research hand in hand with applied research, R&D, and real world engineering in fast succession. And it had over two dozen scientists who would win the Nobel Prize in their lifetimes on the payroll. That is what you call a dream team.
Thirdly, in a small amount of luck it turned out that some of the research that German scientists had done on neutron moderators and neutron production was faulty. They believed that graphite was a poor moderator, probably because the samples they used had large amounts of boron (a neutron absorber) in them. For this reason they ruled out the use of graphite in reactor designs, though it is actually one of the most superior materials for that purpose. This contributed to, but was not the sole reason for, the design Fermi chose for the first reactor (Chicago Pile 1) to be "drops" of Uranium surrounded by graphite, a pretty optimal design.
Fourthly, it had the full backing of the government who financed it well. Compare that to Japan's small scale experiments, Germany's varying level of resource allocation to nuclear research, and Russia's pretty well founded but back burner bomb project during the war.
Not only was the Manhattan Project well funded and resourced but it was exceptionally so. In any other similar program you would expect the scientists and decision makers to choose the best design, decide what research they need to do prior to making certain critical decisions, and then to proceed along their plan. The problem with this in the context of building a bomb during WWII is that it would have been too slow, or not work at all. Remember, the phenomenon of nuclear fission was discovered in 1938, the Manhattan Project began only a few years after (and less than a decade after the discovery of the neutron in 1932). They simply did not know what they did not know, they were flying in the dark. And in fact the most promising design for Uranium separation, centrifugation, turned out not to be workable at the time. It was only decades later with significant technological advancements combined with massive innovations in the design of the systems (the Zippe centrifuge developed by German scientists working in the USSR) that centrifugation became a truly feasible method of enriching Uranium.
But the Manhattan Project didn't work that way. Instead it was a shotgun approach, or a zerg rush. Everything was tried, what didn't work was abandoned, what worked or worked partially was used to its fullest extent, and forward momentum was maintained always. At the start they looked at three possibilities for producing the fissile fuel for a bomb: Uranium-235 isotopically enriched from vast quantities of natural Uranium, Plutonium (predominantly Pu-239) bred from Uranium bathed in the neutron flux of a functioning Uranium powered fission reactor and then chemically separated from fuel rods, and Uranium-233 bred from Thorium placed in a Uranium powered fission reactor then chemically separated from the Thorium. Despite some early promising data on U-233 as a fissile material it was comparatively very disadvantageous to produce next to Plutonium so it wasn't seriously pursued during the initial rush to build the first bombs.
Having candidate fissile materials in hand there was still the matter of producing them in quantities and designing devices which could use them. For Plutonium they had fission reactors and chemical separation, fairly "straightforward" even though a ton of difficulties existed along the way. For Uranium enrichment they pursued multiple concepts: the aforementioned centrifugation, thermal diffusion, gaseous diffusion, and electromagnetic separation. Each of these projects were fraught with difficulties, failures, and roadblocks that needed to be overcome. After each of these systems started getting up to speed they very cleverly decided to hedge their bets by feeding the output of one into the next (thermal diffusion into gaseous diffusion into electromagnetic separation), which would more reliably produce large quantities of HEU faster than allowing each plant to reach its potential independently.
Meanwhile, project scientists were studying the fissile materials being produced in quantity as rapidly as it became available, to precisely determine its properties to inform the design of the bomb devices. In mid-1944 it was found that reactor produced Plutonium was entirely unsuitable for gun-assembly designs due to its high spontaneous fission rate (which could induce a pre-detonation "fizzle"). An entire bomb design plan, the gun-assembly Plutonium bomb, had to be scrapped and enormous effort had to be plowed into implosion assembly designs (which were significantly more complicated).
Ultimately the Manhattan project produced two fuels (out of three tried), three enrichment processes (out of four attempted), and two bomb models (after scrapping one). And, incidentally, the design of the Uranium gun-assembly bomb was considered so fool proof that the first test was its first use, on Hiroshima.
It borders on inconceivable that any other nuclear weapons development program operated by any other country in the world during WWII and operated at essentially any lower level of intensity or enthusiasm would have been capable of building a bomb by mid 1945. It was the combination of the intellectual dream team, vast industrial resources, enormous enthusiastic government support, the "zerg rush" model of operating practically half a dozen separate development programs simultaneously, plus a tiny bit of luck (the graphite) which allowed them to succeed.
While watching a movie, someone suggested that Heisenberg didn't want to succeed in developing a nuclear weapon for the Germans.
There is no real evidence that Heisenberg actively sabotaged the German program. Even Heisenberg didn't claim this, ever. All good evidence points to the fact that the German program didn't get far because nobody thought it would be super successful — that they couldn't actually hope to accomplish such an end given the constraint of the war, and that their resources were better spent elsewhere. And this is probably the correct interpretation; even if you imagine an all-out German nuclear program, it's hard to see them being successful, given the difficulties imposed in terms of resources, manpower, and the very important fact that the US and UK were actively bombing them and deliberately targeting anything that looked like it might have to do with a nuclear program. It's hard to imagine them being very successful under those conditions; Heisenberg's interest has little to do with that (and he was not the only person in charge of the program, in any case).
Was American success due to the will of the people involved? The resources allocated? Sheer luck?
"Will" is a tricky term (and slightly problematic in the context of World War II...). But there's something to be said for it. When General Groves was put in charge of the American bomb effort, he didn't really want to do it — it seemed like a crap shoot. When it was made clear that he didn't have a choice in the matter, he was indeed quite determined to make it work. To this end he secured one major guarantee: that the Manhattan Project would receive the highest priority rating of any US wartime project, and as a result he could requisition the kinds of resources (material, financial, and human) that he felt he needed to have, without opposition.
This kind of commitment (a better term than "will") was absolutely required for the US to develop nuclear weapons on the time scale that they did. Even today, the US program is still the fastest program in terms of order to make the bomb (late 1942) to actual produced weapons (summer of 1945) — 2.5 years. No other national nuclear program comes even close.
How'd they do it? The resources, which were a result of the aforementioned commitment. High quality talent definitely helped — the US had some of the top scientists and engineers in the world working on this project (even many of the "junior" people would later go on to win Nobel Prizes — Feynman, Alvarez, Ramsey, etc. — much less the "senior" talent like Fermi, Oppenheimer, Urey, Lawrence, Urey, Bohr, Chadwick, Szilard, etc.). It was also very well-organized, a factor that historians and political scientists have identified as being essential to successful completion of a nuclear program: Groves was a highly-effective, highly-efficient, highly-centralizing individual who managed the program very well towards its completion. He was aided in this by extremely competent administrators (Vannevar Bush, James Conant), good scientific aides and advisors (Oppenheimer, Tolman), and given essentially a blank check by Roosevelt and Stimson, both of whom also actively fought off attempts to audit or interfere with the program (by people like Senator Truman, or even future Secretary of State James Byrnes, then at the War Production Board, to say nothing of other generals in the military who resented that this unknown program had such high priority).
People often cite the money as the main resources ($2 billion USD 1945, which is some $30 billion USD today), but it's easy these days to be unimpressed by that, since we spend billions all the time. The labor involved is arguably the more impressive number: over 500,000 people worked on the Manhattan Project at one point or another (never all at the same time; there were high rates of people leaving — the peak workforce was around 120,000 people), which is almost 1% of the entire US civilian labor force during World War II.
All of this allowed to make multiple, redundant approaches to the production of the bomb, which was enough to guarantee that a few of them would work.
Even with all of that labor, resources, and scientific brainpower, though, I do think one can put some stock in something like "luck." They only would have had to have been delayed by a few months for World War II to have likely ended some other way without the bombs being ready. Anyone who has attempted to run any kind of project knows how easy it is to be off by a few months. Again, it is still the fastest bomb program in history, even as it was the first one and they were making everything from scratch.
To go full circle: can we imagine Germany having done anything like this, much less under bombardment? It's hard to fathom, even if they had done everything "right," which they did not.
On the German program, Mark Walker's Nazi Science is very readable, but his German National Socialism and the Quest for Nuclear Power is more detailed, and outlines exactly how small the German nuclear program was at all points (never more than a few thousand people worked on it, so it was a factor of 100X smaller than the American program). On the Manhattan Project, they are many books (like Rhodes' The Making of the Atomic Bomb) but if you are interested in something that combines the organizational and the technical, B. Cameron Reed's The History and Science of the Manhattan Project does an extremely good job with both, in a very compact volume. On the specific role of Groves (which is often understated at the expense of the scientific luminaries), see esp. Norris, Racing for the Bomb, which is an excellent biography.
There was an excellent article from /u/restricteddata about the secretly taped reactions of the German scientists when the news of the atomic bombs came out, as well as Heisenberg's own denial to an article claiming that he had sabotaged the nuclear research.
Of course there is always room for more discussion.