But they decided not to tell the scientists who had only agreed to build an atomic bomb because that seemed the only way to stop the Germans from doing it first and using it first to win the war.
These British leaders had also come to the same conclusion about their own Atomic Bomb program taking too many scarce resources as well.
(Like the mostly emigre Jewish scientists pushing hardest to build an Allied A-Bomb, the British had been originally willing to commit all the needed resources and more, if it seemed that Germany might actually make The Bomb ---- and make it first.)
But if Germany couldn't pull it off in time to influence this war, it seemed reasonable to put the British program on the back burner , keeping Britain's hand in, but no more.
They didn't tell the atomic scientists this, but they told Washington.
But the moment the British signalled these joint -and obviously closely related- conclusions to America's leadership, a paradoxical response happened.
Washington's military-political-scientific elite had long been cool to seeing the Atomic Bomb as a weapon of this war and hence something worth diverting billions in scarce war resources towards.
Indeed that elite - centred in and around Vannevar Bush's OSRD war weapon agency - had even been skeptical The Bomb would become a weapon in any future war.
But now the possibility of an all-powerful strategic weapon so complex and expensive only the US could afford to build it and still play a full part in the current war made it an exceedingly attractive diplomatic weapon for what Washington saw as its new role as the world's new global policeman in the post war world.
America decided to share (and even there reluctantly) only information in areas of atomic bomb research that Britain and Canada were already equal scientific partners in.
Meanwhile America would command by force majeure all the vital Canadian resources crucial to making The Bomb, denying them both to Canada and Britain.
Canada's uranium wasn't vital, just useful, but its processing plants and expertise for making uranium oxide and heavy water were vital in the sense that replicating them might add nine months to the path to the first successful atomic blast, possibly taking it outside the timeline of the war.
(The OSRD was praying that the Germans and Japanese didn't surrender too fast, because then Congress would instantly stop spending billions on a merely possible atomic bomb.)
I have never accepted the conventional explanations as to why America suddenly pushed their atomic bomb program into high gear at the same time they cut out Britain from the originally-planned team effort.
Lessons from the attractiveness of difficult-to-synthesize penicillin
But as I came to see how synthetic penicillin's very difficult nature made it more - not less - attractive as a secret war weapon to the OSRD, I began to see how the same could apply to the A-Bomb as well.
You see, cutting out Britain from A-Bomb research during the war would not make any sense at all, if Germany had a well developed A-Bomb effort.
Because at the end of the war, all the leading Allies would have an equal moral right to profit off of Germany's scientific treasures.
Britain and Russia would then learn - from the Germans - the 'devil in the details' knowledge to take the atomic bomb from university lab theory to successful factory production.
Believing that neither Britain and Germany would make an atomic bomb easily (let alone Russia, Japan, Italy and France) feed Washington's early-senility induced delusion that the atomic bomb secret really existed.
That 'secret' only existed for a short period - during the war itself - rather like the DDT secret.
Still, a useful period if it really was to be a war-ending weapon ( which no one in Washington believed) but useless if post-war secrecy was key to its success as a diplomatic weapon.
Poor old Washington and poor old London and poor old Berlin.
Always forced to fight a three front war.
Two minor fronts (against your darkest enemies) and then a major one ---- against your dearest friends......