What happened to Russia’s government between World War 2 and pre-Cold War that the United States went from supplying them against Germany (a massive amount) to the highest tensions could get without actually snapping? Under the Lend-Lease, we sent them somewhere around $12 billion worth of aid, and now we’re a button away from full scale war? Looking at Britain’s Operation Unthinkable, if the Western forces were that worried about the Soviet Union, why didn’t we leave them alone when Hitler moved east?
The last question can be answered by referencing the bare and obvious facts. By the time Germany invaded the Soviet Union, the UK and the commonwealth had already been at war with Germany for a year and a half. And Germany declared war on the USA in December 1941, not the other way around.
Resources.
Had Hitler been able to conquer the Soviet Union, there was a vast array of natural resources available for the Germans. Natural Gas, Iron Ore, Coal, and almost everything needed to keep building weapons. Not to mention the Soviet weapons and military resources that were already built.
When the German Army originally invaded the Soviet Union, Stalin ordered a scorched earth policy upon retreating to the interior. As German forces advanced, anything of value that could be used by the Germans was either shipped further into the interior of the Soviet Union or made completely unusable by German forces.
There were also Nazi collaborators in Russia. In great force. The Soviet Union was not a politically stable nation. There were many fighting for the liberation of the Soviet Union from Communism and saw the Nazi regime as the lesser of two evils. So had the Germans successfully occupied the Soviet Union, there would have been more people to fight against the Allies. There were also Nazi divisions made up completely of Soviet Soldiers, there wa also a Soviet SS unit.
The leadership of the Allies saw it as the Soviet Union being the lesser of two evils. The fall of the Soviet Union would have been an extension of the length of the war, it may have also been a tipping point for the Nazis.
The Lend-Lease Act was established with the primary goal of defeating Nazi Germany.
The US-Soviet Alliance was a rocky one too. There was actually in-fighting during World War II.
The common goal amongst the Allies was to defeat Nazi Germany. Beyond the common goal, the relationship between the Soviets and British/Americans wasn't very strong.