The Truman Doctrine was a major turning point in US foreign policy for at least two reasons.
First, the doctrine can be seen as the beginning of American participation in the Cold War. The Cold War was, of course, to dominate American foreign policy up through at least 1991. The Truman Doctrine marked the first major step towards the containment of communism as the major focus of American foreign policy.
Second, the doctrine can be seen as a move away from isolation and the beginning of the American role as the “policeman” of the world. For decades before 1947, the US had not typically involved itself in the affairs of the world. Now, the country was going to completely change its approach. Instead of remaining isolated or only intervening when something was clearly in its immediate national interest, the US was not going to try to maintain order in the world as a whole. This was a huge change from what the US had been. Now, the US was the world’s preeminent power and the leader of the free world.