How effective were the efforts of the United States to win the Cold War during the late 1940s and the 1950s?
American efforts to win the Cold War during this time period were not effective. If they had been effective, the Cold War would have been over. However, they were effective in the sense that they kept the US in a relatively good position relative to the Soviet Union. Since the US ultimately won the war by outlasting the Soviet Union, we can say that these early efforts helped to set the groundwork for later success.
On their own, these efforts did not win the Cold War. By the end of the 1950s, the two sides were close enough to equal that John F. Kennedy could credibly criticize the Eisenhower Administration for letting the Soviets get ahead in the Cold War. This clearly shows that the US had not won the Cold War by any stretch of the imagination.
However, the foundation had been laid for the eventual victory in the Cold War. The Marshall Plan, for example, had helped to ensure that Western Europe (and the US) would be economically successful. The occupation of Japan had done the same for that country. American actions in the Berlin Airlift and the Korean War showed American resolve against communist efforts to expand. These were the basic elements that would allow the US to eventually win the Cold War. Thus, American efforts in the late 1940s and the 1950s were not immediately effective, but were important in the long run.