Given these two options, America was clearly responding to Soviet expansion. At the start of the Cold War, there was really not any American aggression going on. The US had not made any moves that were directly hostile to the Soviet Union. By contrast, the Soviets had taken control over all of Eastern Europe and were clearly trying to impose communism on those countries.
However, I would argue that the Soviets were responding to something -- they were responding to Russian history. So it's not as if they were simply being aggressive and wanting to expand. The Soviet Union had good reason to want buffers with the West. Their country had been invaded (and almost conquered) both by Napoleon in the 1800s and by Hitler in WWII. Because of this, there was ample reason for them to want friendly countries around them.
Overall, then, I would argue that the Cold War was started by America's (understandable) reaction to Soviet expansion which was itself a (understandable) reaction to what had happened to Russia in the past.