“how did the united states and the soviet union go from allies during the second world war to enemies engaged in a cold war? ”

After WW2, the Russians tried to spread communism to other countries and became nuclear capable. Being as how America is a Capitalist country And Russia is a Communist country they are total opposites resulting in some conflict because we didn't want communism to be spread. Russia became nuclear capable and showed off what they made and had, so we showed off what we made and had, and both countries started making, bigger, badder, and more powerful bombs. All to prove to the world who the real "powerhouse" is. I HOPE I HELPED!!

Why do you have to ask such hard questions.