The secondary analysis of the question is an interesting one.  I think that isolationism did depart with the advocacy of the Truman Doctrine andfollowing with the one followed by Ronald Reagan.  Both sets of philosophies clearly identified "the enemy" as the Soviet Union and the spread of Communism.  In this light, there was a clear and focused belief that where the threat of Communism lingered, United States action was needed and demanded.  One of the major effects of this was to define foreign policy in one light in being anti- Communist.  For example, when the Soviet Union experienced challenges with the Afghanmujaheddin forces, the United States immediately came to the aid of these insurgents.  One of these lower level elements was one Osama Bin Laden.  With a singular focus and definition of being anti- Communist, America failed to look past this threat and examine the other, moremulti -polar threats that could exist.  If someone would have indicated to American leaders that Bin Laden would be the chief focus point of the newmillennium's war against the United States, perhaps there would have been some level of insight to not so openly aid and assist him in his battles with the Soviets.  The strict and limiting definition of foreign policy as seeing the Communists as the only enemy against the United States impacted envisioning new paradigms and approaches to US threats.

When you say "this new direction," do you just mean the Cold War in general?  If so, these two doctrines (you can argue) began and ended the Cold War.

The Truman Doctrine can be seen as the start of the Cold War.  It said that the US would help any country that needed help in repelling a communist attempt to take it over.  That set us up in opposition to the Soviet Union and started the Cold War.

The Reagan Doctrine was that the US should aggressively build up its military.  That would force the Soviets to do the same.  Since they had so much less money than we did, it would hurt them.  Reagan fans say that this is what led to the collapse of the USSR.