Let's get to the second part when the war is actually over for the Russians.
In 1945, the Soviet Union, true to its obligations to its allies, declared war on Japan and defeated its Kwantung Army in Manchuria.
In May 1945, Britain plans Operation
Unthinkable to attack the Soviet Union.
In 1949, the United States developed a plan for Operation
Dropshot.
"
Dropshot included mission profiles that would have used 300 nuclear bombs and 29,000 high-explosive bombs on 200 targets in 100 cities and towns to wipe out 85 percent of the Soviet Union's industrial potential in a single stroke." - Wikipedia
Bloody Russians did not appreciate such benevolent steps of their good Allies and also began to work on nuclear weapons.
That's how it happened, Bobby