What is a historical example where US foreign policy was based on exceptionalism?

There are a number of examples of this, but perhaps the most notable one from the 20th century is the Marshall Plan, in conjunction with the Truman Doctrine. The idea of American exceptionalism is, essentially, the belief that America stands apart from other countries on the world stage and therefore has more of a responsibility to provide aid to other countries. In modern foreign policy, we also see the reverse theory imposed: the USA does not accept foreign aid from other countries, even when it is needed, because of the belief that America should be seen as the "giver" on the international stage. In 1947, American exceptionalism led to the belief, outlined in the Truman Doctrine, that it was the responsibility of the United States to contain the spread of Communism in Europe, and as a result of this, the Marshall Plan was born. 
Europe in 1947 was utterly ravaged by war. Germany had suffered particularly badly from the war, but within the constraints of the Marshall Plan, it was less deserving of American aid than, for example, the UK, because it had also caused the war. The Marshall Plan was intended to prevent Communism spreading in that it offered aid to European countries to prevent their turning, as Czechoslovakia did, to Communism. As such, it was meant to disincentivize the poverty-stricken European countries from moving, politically, to the left as a measure against their suffering: instead, they could turn to the US, if they maintained the "right" politics. The Marshall Plan, effectively, was an example of the US utilizing its financial clout to ensure its own ideals prevailed on another continent.  
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