As General Patton knew, Russia was never the ally of America. Since Russia was never an ally, there was no reason to have allowed Russia after WWII to divide Germany and to access some of the "brain power" which helped with the development of nuclear weapons. And, had the U.S. done what Patton suggested--namely, go into Russia at the end of WWII, the U.S.S.R. would not have developed as it did. Therefore, there would have been no Cold War. Blame goes to the U.S.
The posts up to this point are good, but I've always wondered, when looking at a simple timeline of history, if the US didn't play a leading role in the intensification of the Cold War. When the three western Allied powers (the US, England, and France) created a unified German currency in 1948, for example, the Soviet Union responded with the Berlin blockade. Similarly, if I recall my history correctly, the Federal Republic of Germany (West Germany) was established before the German Democratic Republic (East Germany). At least with these examples, the Sovets seems to have been reacting to Western (US-led) actions. I've always wondered if things might have ended up differently if those western powers hadn't unified the (West) German Mark or unified their sections in the immediate post-WWII era.
Of course, that's just one example presented by me (a non-historian) in simplistic terms. I'm sure that a global view of the Cold War will show give-and-take on both sides.
Equal responsibility is the answer I would go for in response to this question. Both the USA and the USSR did nothing to try and stop the Cold War, and did everything to intensify it. It is impossible with such a question to calculate who is more responsible, it is such a difficult decision. In the end, it was a real game of brinkmanship that made popular opinion and perceived victories against each other incredibly important.
I agree with previous posts - both the United States and the Soviet Union contributed to the stand-off that we know as the Cold War.
In the longer view of history and international relations, deciding which country started the Cold War isn't as important as looking at what we can learn from it and its aftermath. If politicians and governments could understand the motivations that led to the Cold War and could find different ways of expressing concerns about national security and/or pride, the economic and human ramifications could be significantly altered in years to come. The concerns are still present and valid - the key is in finding less threatening ways to express them.
At the end of World War II, Joseph Stalin told the Soviet people, "the war against fascism has ended, the war against capitalism has begun." At the same time, President Harry Truman argued against allowing Communism to spread. The cold war was as much about ideology as it was political dominance. There was tremendous misunderstanding and misinformation on both sides; the Soviets painting the U.S. as capitalist aggressors, and the U.S. depicting the Soviets as "godless" and an evil force determined to take over the world. Bottom line, I must agree with bullgatortail: the U.S. and the Soviet Union were both responsible for the Cold War.
I believe the U.S. and the Soviet Union are equally responsible for the Cold War that occurred immediately after World War II. They emerged as the two most powerful nations following WWII, and both countries can be faulted with antagonizing the other. The U.S. was unhappy with the increased spread of communisim in many of the Eastern bloc nations, and Americans became increasingly paranoid of the possible threat of atomic or nuclear retaliation. Of course, I suppose Germany also deserves some blame for initiating WWII and the effects of the Cold War that followed.
This is very much a matter of opinion. You could argue this in at least two ways.
You could say that the Soviet Union was responsible for the Cold War because of their aggressive actions in Eastern Europe. If the Russians had not tried to make all of Eastern Europe into puppet states of theirs, the Cold War need never have happened. In this view, the USSR was responsible because it refused to allow Eastern European countries to choose their own destiny.
You could also argue that the US is responsible because it overreacted to Russian actions. The US felt that the taking of Eastern Europe was a prelude to a broader effort to take Western Europe and eventually the whole world. You can can argue that the US should have seen that the USSR just wanted a buffer zone between it and the West. If you believe this, the US is responsible for the Cold War because it had an irrational fear of the Soviets.
My take is that both had reasons to enter a Cold War; however the Russians forced the issue the most. During WW2, they seized control of nearly every recognized Eastern European country plus portions of Finland, Germany and Korea. After WW2 they installed puppet governments in them or openly occupied them. Blame = Russians.
Continuation of WW2 by the US going into the USSR would not have guaranteed victory or the exclusion of a Cold War. So you can't put blame on the US for a theory of something that might have happened IMO. It's certainly an interesting possibility though.
During World War II it seems as though we knew (or should have known) we were in "bed with the enemy" but needed the Soviets to maintain the Eastern Front of the war while we attacked on the West to liberate the Western Front. I think we underestimated the brutality of Stalin and any future plans he might have on a global scale because we were too focused on the immediate issue of Hitler and Nazi Germany. Once the war ended we quickly realized we needed to reinforce our efforts to minimize Stalin’s power in Western Europe. Maybe we overreacted but given the length of time we allowed Hitler and his unorthodox politics to dominate Europe we surely did not want to make the same mistake twice. However, both sides were guilty in perpetuating and escalating the conflict.