My understanding is that the US was perfectly willing to throw some money and time at almost any sort of military project during WWII (Project Pigeon, anyone?) but I've never been able to get a clear picture of just how long it took for the Manhattan Project to cross the threshold from theory to "This will totally work!"
Was the Manhattan Project at first considered some sort of a fool's errand, or was the theory behind the atomic bomb solid enough that there was no doubt a bomb could be built, with enough time and resources spent on making it happen? What milestone moment proved that atomic weapons were possible? Was the threat of the UK, Russian and German atomic programs enough to ensure that the Manhattan Project would continue?
Thank you.
The Manhattan Project qua Manhattan Project started in late 1942. This was the "let's actually build a bomb" phase of fission work (the earlier phases were a "can it work?" sort of thing).
As it began to hoover up more and more resources with no apparent output, especially as the war in Europe was drawing to a close, there were those in Congress and elsewhere who questioned it. There were attempts to "audit" the project, e.g., to determine whether it was worth the effort. My favorite is from Senator James Byrnes, who later became Truman's Secretary of State (and a big fan of the atomic bomb), who in March 1945 sent a letter to President Roosevelt requesting an external assessment of the project's worth:
I understand that the expenditures for the Manhattan project are approaching 2 billion dollars with no definite assurance yet of production.
We have succeeded to date in obtaining the cooperation of Congressional Committees in secret hearings. Perhaps we can continue to do so while the war lasts.
However, If the project proves a failure, it will then be subjected to relentless investigation and criticism.
I know little of the project except that it is supported by eminent scientists. Even eminent scientists may continue a project rather than concede its failure. Also, it may be feasible to continue the experiment on a reduced scale.
In any event, no harm could come from an impartial investigation and review by a small group of scientists not already identified with the project.
Such a review might hurt the feelings of those now engaged in the project. Still 2 billion dollars is enough money to risk such hurt. I think Dr. Bush would be glad to have two or three scientists whose pride was in no way involved in the project, check it and report that results justify continuance of the project.
A favorable finding would justify continuance, regardless of future success or failure. An unfavorable finding would at least indicate the need for further justification by those who are responsible for the project.
In any event, it would be clear that we were mindful of the tremendous expenditure of men and materials.
It didn't go anywhere. The Secretary of War (Stimson) didn't want any such review. He asked Groves about it, and Groves pointed out there was practically nobody in the USA who was qualified to evaluate the project who was not already part of the project.
Anyway. Could it have been cancelled? Only if the secrecy was broken or the Secretary of War or Roosevelt changed their minds. On the latter points, there was little chance of that — they were convinced. On the former point, there were some places where Congressmen, newspapers, even scientists seemed to threaten the secrecy, but in the end the dam held enough that it never got subjected to major scrutiny. This avoidance of outside opinion (who would have potentially judged the expenditure of resources on such a sci-fi idea to be unwise) was one of the many reasons they kept things as secret as they did. It was deemed by the scientists the sort of thing that would be hard to compellingly explain to a Congressman. They did eventually bring a few Congressmen in on the project so that the budget could be moved through easier (as the Byrnes letter references).
As a small point, the official motivation of the early Manhattan Project was against Germany, not the UK (who was a partner) or Russia. It gradually shifted towards use to end the war with Japan when it became clear that the Germans were not building a weapon themselves.