Nationalist China with Nukes in 1945
The Japanese have a sudden burst of brilliance and build Liquid Thermal Isotope separation plants in northern Korea starting in 1937, just after the neutron enhanced fission phenomenum is discoverd.
The USSR learns that the Japanese are building Liquid Thermal Isotope separation plants in North Korea by tapping the underwater telegraph cables with a special sub in 1939. We did build cable tapping subs in the fifties and there are books about them. In this ATL the Russians do it first.
They build up their army faster and when the Germans invade Poland they invade Manchuria instead of Finland, liberating the Chinese. They use their parachute corps, the biggest in the world, and seize the north Korean facility intact, following up with a conventional armored attack across Manchuria. They have a cargo ship docking in the Korean harbor with more troops on board and they reinforce the attack force with supplies and more troops. This gives them the hydroelectric facilities and the LTD plant but it still takes two years to deliver the fissionable materials in adaquate quantities.
The war in Europe is still going on. Hitler dumps his treaty with the Japanese because with the rest of Europe allying against him he has enough to worry about.
The Chinese-Japanese war ends by June 1941 because the Russians blockade Japan, having started by sending their subs into Japanese harbors in a sneak attack and having their small fleet trailing Japanese merchant ships to sink them, having sent them out previously to do just that. Japan is furious, but what can they do? The Russians have a large airforce and sub fleet just next door, and a surface navy that essentially bans the Japanese merchant fleet from the Atlantic, the Indian Ocean, and the eastern Pacific.
The Japanese don't have any friends and after Nanking they are susceptible to propaganda attacks so they don't get any friends. They can't reinforce China without unacceptable casualties, they lose air supremacy over China, the communist Chinese are massively reinforced by a new railroad across Mongolia and can recruit more successfully with plenty of food and supplies, they can't get credit from Europe because Europe is fighting a war, they can't import oil because the Russians keep sinking their tankers, they can't get more ships from Europe because the German subs are sinking the European tankers, the Russian airforce is mining their harbors with long range aircraft flying at night, etc.
The USSR does better in their war with the Germans if the Germans conquer Europe anyway because in this war they have lots of experienced troops to match the experienced German troops, lots of troops from their Chinese allies, not to mention that the Germans lose Berlin in July of 1941 to a nuclear attack. This sort of disrupts the German alliances.
So the US gets very enthusiastic about LTD plants and builds lots and builds some more dams to get more electricity, too. Instead of the Alaska road and pipeline we build the Churchill Falls hydropower unit in Labrador and a railroad there past the iron ore deposits that we stopped at in OTL. We aren't fighting WWII ITTL, so we make much faster progress.
The 1942 peace treaty leaves Europe demilitarized and nuclear free, and the US and the USSR confront each other in a bipolar world. The world has had a much less bloody and expensive WWII. Especially Russia!