Why was the Marshall Plan implemented while U.S. President Harry Truman was in office at the end of WWII?

Just think about what could have happened to Europe. We may have ended up with a third world war.

The rebuilding of Western Europe was to stop the spread of Soviet influence and Communism.

Most of the money used to rebuild western Europe and Japan, was given in the form of grants, not loans, and was not expected to be paid back. This was, as BlueEyedAlien said, to prevent the spread of Communist influence. It was also to foster economic stability. Unofficially, it was also to prevent another Germany, a country so badly punished after WWI by the Treaty of Versailles that German money became useless and people starved. Many believe this led to Hitler's hatred, and believe it to have been one of the causes of WWII. While it is a gross over-simplification, the fact that Hitler made France surrender in the same boxcar where Germany surrendered twenty-odd years before makes it clear he was thinking about it.