•how were foreign affairs guided by national self-interest?

Country's foreign affair policy also called foreign relations policy, consists of self-interest strategies chosen by the state to safeguard its national interests and to achieve its goals within international relations milieu.

In general, foreign affairs are always guided by self-interest, since a country would not want to get involved in the affairs of another without there being some domestic benefit. One such example would be the US involvement in Vietnam, since it was thought that this would discourage the spread of communism, which was the enemy of capitalism. Another example would be the US involvement in WWII, since the spread of fascism was frightening to many.