How was the Truman Doctrine different from the traditional American policy of isolationism?
This was different because it had the US intervene in areas where it had no direct national interest.
In general, the US had tried not to get involved in the affairs of other countries unless there was a direct connection to US security. With the Truman Doctrine, this changed. The Truman Doctrine had the US commit to helping any country that was threatened with being taken over by communists. This even applied to places like Turkey and Greece that were far from the US and had no clear connection to US security. In this way, it was very different from the policy of isolation.
The Truman Doctrine was different from traditional American isolationist policies because it committed the United States to getting involved in conflicts that were not clearly relevant to American security.
The previous American foreign policy (at least since the 1920s) had been one in which the US tried to avoid getting involved in conflicts that were not obviously connected to American interests. The Truman Doctrine did away with this idea. It stated that the United States would help any country that was in danger of having its democracy overthrown by communists. This would be done even in countries like Greece or Turkey that had no clear connection to the United States.
The new doctrine, then, committed the US to preventing the spread of communism even in places not immediately important to the US.