Okay, so I just finished reading Freedom Betrayed by Herbert Hoover, which was great and raised a lot of questions that I'd never thought to ask, but obviously he's got a huge ax to grind, so I wanted to throw this question out here. Two parts to this:
The obvious answers are (1) the Russians already occupied eastern Europe, and (2) Manchuria was the price for the Russians entering the war against Japan. But at least to Hoover, these arguments don't hold water, because (1) American lend lease supplied the Russian war machine, and if we stopped sending aid to the Soviets, they weren't occupying a damn thing, and (2) we didn't need any help in the Pacific; the Japanese were begging for surrender terms for some time before Hiroshima, and as it turned out in fact, the Soviets were at war with Japan for all of two days before they surrendered. For that they wound up with Port Arthur, a bunch of islands, and all of Manchuria? Seems like a sweet deal if you're Stalin, but why would the Americans go for that at all?
That's the Cliffs Notes version, I guess. So how much of this is revisionist BS, how much is hindsight, and how much was legitimate and we should have known better at the time? And above all, why?
The post war order in Europe was agreed upon by the big 3 at Yalta. The Polish question was actually one of the biggest sticking points of the agreement. The English wanted the Polish government-in-exile based out of London to be the post war government while the Soviets wanted the Lublin Committee (communist puppets) to be the government after the war. Roosevelt was pretty ambivalent about the whole thing. He wanted a democratically elected government in Poland obviously, but he wasn't willing to sacrifice good relations with Stalin over it. By the end of Yalta Churchill had wrung a promise out of Stalin to permit a democratically elected government to form in Poland and all nations in Europe.
However, as the war came to a close Stalin ran roughshod over the agreement reached at Yalta. Communist led governments were quickly set up in Romania, Hungary, and Poland by the Soviets. In fact, the Soviets helped instigate the Warsaw Uprising broadcasting appeals for the Poles to attack the Germans. Once the uprising started the Red Army, which was sitting directly outside of Warsaw, did nothing to help. The Germans exterminated the Polish Home Army which surely would have resisted the Soviets creating a puppet state out of Poland.
Churchill resisted all of this but did not find much support from America. Just Roosevelt started siding with Churchill against Stalin he died. Truman, new to The Presidency, did not want to alienate Stalin. The fact of the matter is the Western Allies could not do much about the governments set up in Eastern Europe. They could never reach it before the Soviets did and attempting to penalize the Russians by ending lend-lease was idiotic. Stalin would have almost assuredly withdrawn from the war leaving the Western Allies to finish up Nazi Germany on their own.
The only places where America and Britain actually had a shot at stopping the Communist advance were Czechoslovakia and Germany. In the closing days of World War II, after America and Britain had crossed the Rhine and Russia had crossed the Oder, the generals of the Wehrmacht started disobeying orders en-mass. Their biggest priority was saving as many Germans as possible from the Communists in the east. In fact, General Wenck, who was in charge of the German 12th army holding the Elbe River against the West, turned his soldiers around -against orders- and marched east to fight the Soviets. This and other moves by rogue generals meant the American armies had clear routes east to Berlin and Prague. Bradley and other US generals were requesting permission to continue advancing past the line agreed upon at Yalta. It was Eisenhower who nixed this. He chose to let Berlin and Prague fall to the Russians honoring the commitments made at Yalta. The Russians had no such qualms. When Allied generals arrived to sign the second German surrender in Berlin the Russians delayed the signing for hours to give them time to steal or destroy everything of value in West Berlin.
As for the Eastern Theater, the Joint Chiefs of Staff wanted Russia to join the war to fight the 700,000 strong Kwantung Army in Manchuria. While the US military and intelligence services had some information that this army existed only on paper (which it did) the Joint Chiefs believed this was a complete army and did not want to sacrifice American lives to fight it.
To sum up, the Soviet Union got control of Eastern Europe because they liberated it and any attempts to stop them would have resulted in their withdrawal from the war, which would have resulted in the deaths of many more western soldiers. Stopping them militarily was out of the question since after almost a decade of War the citizens of America and Britain would not have supported immediately attacking our ally to guarantee free elections in countries far away. The Soviet Union got control of Manchuria and Port Arthur because the Joint Chiefs were worried about an army that didn't exist. I highly recommend reading "The Last 100 Days" by John Toland for more information. I drew most of the information for this comment from this book.
You probably should start from this. How important was lend-lease to the Soviet war effort!
Then I'd recomend you to watch this, if you not familiar with ww2 in general. http://www.fallen.io/ww2/
Soviet operation in manchuria was also discussed recently. https://www.reddit.com/r/AskHistorians/comments/62743u/ive_heard_people_say_that_the_russian_invasion_of/
The Japanese were trying to get Stalin to help them get a negotiated surrender as opposed to a complete unconditional surrender. Stalin was stringing them along though since he intended to invade. Until the A bombs were dropped we still had plans to invade the mainland so we wanted Soviet help.
Racing the Enemy by Tsuyoshi Hasegawa has an excellent up to date review of the end of the war with access to Japanese archives. The Japanese knew they had lost but had hopes of keeping the regime in power and negotiating surrender. FDR had said only an unconditional surrender would be accepted. It was after the A bombs AND the Soviets invaded that the Japanese Emperor decided to surrender unconditionally.
Also as already noted, we couldn't kick the Soviets out of eastern Europe even if weren't already involved in finishing the Japanese. They had troops in those countries and we occupied in the Pacific.