When the allies came together, it was decided to let Stalin have control of the eastern front. This was before we knew about the Final Solution and concentration camps.
I understand wanting to have a second front but why weren't controls put in place so that Stalin would have to relinquish the territory he conquered like the allies did?
In hind sight, he took over more territory than Hitler. He was just as cruel, if not more so. What made the allies decide to make him an equal?
There were attempts at controls. This is part of why the United States dropped the atomic bomb on Japan. Yes, there fear of a bloody war for the home islands. However, the Americans were also concerned that the Soviet Union would attempt to overrun Asia as they had with Eastern Europe. Which of course they attempted to do. In August of 1945, Russia invaded Manchuria. The Japanese army stationed there was wiped out in less than two weeks.
So the United States pushed harder for a quick surrender, including concessions that Japan would keep the current Emperor, and that the US would rebuild. By August of 1945, we begin to see what would eventually evolve into the Cold War policies of Containment, the Truman Doctrine, and the Marshall Plan.
Japan also had significant strategic value as for military bases. It was also a modern, industrialized nation before the war so reconstruction went quickly. It paid off in spades when East Asia became a major theatre of the Cold War.
In Europe, the alliance with the Soviets was more one of convenience. Make no mistake that the western Allies were leery of them. But, when any sane man is given the option to bring a quick end to a brutal war, he will always take it. The details can be hashed out later. Besides, breaking the alliance with Stalin could have convinced the USSR to simply keep going. It was decided that Germany would be divided into sectors. Following the war, the Allies, now NATO, made it clear that they weren't going to let Stalin step beyond the Iron Curtain he had been assigned. This was accomplished via the Berlin Airlift, and later the Marshall plan.
Whether containment actually worked is debatable. Communism did expand rapidly during the Cold War despite attempts to stop it. However, most countries had abandoned it when the USSR fell in 1991. As far as bad economic policy goes, communism is the poster child. I think Marx may have taken an overly optimistic view on human nature, and assumed that there would be nobody who wanted to be in charge, or take advantage of the system. Read Animal Farm to see how that happens. Needless to say, it's just economically and politically unstable.