How did new weapons turn the former Allies into rivals after World War 2?

That was caused by the development and deployment by the US of the atomic bomb and a little later, of the hydrogen bomb. Stalin had quickly realized that this development had put the US at a huge advantage, militarily and therefore also politically. Russia's race to develop an atomic bomb of its own was one of the major triggers of the Cold War and the arms race that developed between the Western Allies and the Soviet Union.