How did truman argue that the situation in these two countries was a threat to worldwide political stability?

When discussing the importance of passing the Truman Doctrine, Harry Truman addresses how failing to help Turkey and Greece will have long term negative effects on the world.

In his speech, he argues that a nation that struggles economically could result in a totalitarian regime. America has seen that totalitarian regimes in different parts of Europe has resulted in utter devastation for the citizens. Along with this, Truman argues that if America "falters" in our leadership now, that it would endanger the peace of the entire world, as these countries may be taken over by another country such as the Soviet Union.

Apocrypha is the answer your looking for