Final answer:
The primary motivation for U.S. engagement in foreign affairs post-World War II was to fill the power vacuum left by weakened European nations and to counter Soviet influence, marking a shift from isolationism to active global engagement to secure American interests.
Step-by-step explanation:
The primary motivation for US involvement in foreign affairs during the period after World War II was rooted in the shift from a policy of isolationism to one of active engagement in global politics. The United States began to recognize the necessity of stepping up as a world superpower, given the weakened state of traditional European powers like Great Britain and France. This transformation was driven by various factors including the desire for access to resources, markets, and the spreading of political and economic systems counter to the rise of the Soviet Union's influence.
Initially, America's foreign policy was centered around domestic issues and protecting the western hemisphere from European colonial ambitions. However, by the late nineteenth century, the imperialistic ambitions of European powers influenced the US to also pursue imperialism. The emergence of the US as a superpower after World War II necessitated a permanent change in its foreign policy from isolation to active engagement to maintain global stability and security, particularly to counter the ideological and geopolitical threat posed by the Soviet Union as outlined in NSC-68.