WI no McMahon Act

The McMahon Act was a piece of post-WW2 legislation governing American nuclear policy with the rest of the world. When it was passed in 1946, British access to information from the Manhattan Project was halted, and it effectively ended American and British nuclear cooperation until the late 1950s. It was a slap in the face to the British, and led to the establishment of an independent UK nuclear program.

But in large part, the Act seems to have arisen out of the chaos in the aftermath of Roosevelt's death. Apparently, much of the paperwork dealing with the various Anglo-American agreements on nuclear technology were lost - one of the biggest backers of the bill, Senator McMahon himself, said that if they had known about these agreements, the bill never would have existed.

So, let's say that things are a little more orderly for the incoming Truman administration - the existence of the Quebec and Hyde Park Agreements is known in Washington, and the McMahon Act never takes place. What affects would this have on relations between America and Britain, and the early Cold War?
 
The McMahon Act did a lot more than just making US and UK relations more complicated. It also set the basic structure of the Atomic Energy Commission, such as having it headed by a bipartisan-ish commission of five instead of a unitary director, and making it a civilian rather than a military agency. Preventing or altering the McMahon Act could have lots of fascinating impacts on the development of atomic energy, especially if the military retains effective control over the field.
 
Well if we're positing that it's true about Senator McMahon saying it would never have gone ahead had he known and that the paperwork doesn't go astray the easiest thing to do is simply have a different bill with the Atomic Energy Commission bits in it, nuclear knowledge is still classified up the wazoo but with a get-out clause similar to the 1958 US–UK Mutual Defence Agreement that allows the President to co-operate with certain allied countries. The other, and possibly more interesting, alternative as Asnys mentions is where the bill never happens and things are radically different. Either way, if the British still have access to the Manhattan Project data it's going to make the development of their own nuclear weapons much easier and cheaper.
 
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Does this make control of proliferation easier? Since the Brits & Americans are now even, & there's less latent hostility over the issue, are they more inclined to agree about letting anybody else into the "nuclear club"?
 
Well the problem is Russia is going to develop her own nuclear weapons regardless of what the US or UK do and say, and I can't really see the US ever giving them any help so they haven't got much to bargain with. Likewise the French are going to want in as they still see themselves as a great power and the leading nation of Europe so they feel they need to have it, the only thing that the US has to use as pressure is their Marshall Plan aid and they've got to balance using that against how popular the French communist party was after the war and helping rebuild the country to help defend against the Soviets. If the Russians have nuclear weapons then you're going to need extra points of departures to keep them from giving it to the Chinese, ditto the French helping the Israelis with their program out of a feeling of owing them from the whole Suez Crisis affair. India and Pakistan are a bit easier to influence but not greatly, if India decide they need the bomb then they're large enough to do so provided they feel accepting the consequences is worth it. And once India has it then Pakistan will have to have it. Or maybe I'm just being overly pessimistic. Even in our timeline as it is we've been pretty efficient and lucky at keeping the nuclear genie bottled up.
 
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