The quote below comes from a speech given by President Harry Truman in March 1947:

I believe that it must be the policy of the United States to support free peoples who are resisting attempted subjugation by armed minorities or by outside pressures. I believe that we must assist free peoples to work out their own destinies in their own way.

Source: http://millercenter.org/president/speeches/detail/3343

What threat was President Truman directly addressing in this speech?

nuclear proliferation
communist aggression
anti-colonial rebellions
fascist dictatorships