I realize that it might be a kind of naive question to ask why a country did not attack another country. But wouldn't it have been a huge opportunity for the US to establish a western world order? Moreover, they could have prevented the Cold War and the current conflict in the Ukraine.
The alliance between the US and the Soviets was more of a purpose alliance. They only fought together because they thought Hitler was the greater danger. I believe that it must have been clear that, after the axis powers were beaten, there would be conflicts between the US and the Soviets.
The Cold War was so dangerous because two nuclear superpowers were facing eachother. The Soviets tested their first nuclear weapon not before 1949 though. Also, the Soviets military was weakened much more than the US military in WWII.
So I conclude that 1945, right after Germany and Japans capitulation, would have been the perfect moment for the US to attack the Soviet Union, Eluminate Communism and create a western world order. Why didn't they do it?
There are lots of potential reasons — the United States was, to be sure, tired of war, for example. But it is just worth pointing out it would not be an easy fight to imagine. The USSR by 1945 had taken rough possession of most of Eastern Europe and a nice chunk of Manchuria. They had a very large and seasoned army, and they had reorganized their wartime economy after the disasters of the early war. If the US were to start a war with them, would they have been able to stop the Soviets from immediately taking over much of Western Europe as well?
The major advantage the US would have had would have been air power — atomic and conventional — but the distances into the USSR are non-trivial for a B-29, and the stockpile of atomic bombs that the US had at its disposal from 1945-1950 were pretty small. The US Army Air Forces estimated it would take several hundred atomic bombs to take out the USSR in a "knock-out blow" — and they didn't have those. Even getting them to targets in the USSR would require many foreign bases as "launching points", many of which the US did not actually have in its possession. At the time, it still took multiple weeks (if not months) to assemble and prepare individual atomic strike missions — it was not a "push-button" war just yet.
Separately, if the Germans couldn't push the USSR over despite their early advantages... what would make an American general optimistic they could, too? The gamble would be huge, and timeline would be long, and the casualties would be massive. And the US populace was in no mood for such a thing, and neither were the generals or politicians for the most part.