Under its foreign policy of containment, the United States promised to ?

Under its foreign policy of containment, the United States promised to stop the expansion of an enemy. It has been implemented during the Cold War wherein alternatively it is known to be the Cold War policy of the United States. It aims to prevent the spread of communism abroad.  

Under its foreign policy of containment, the United States promised to keep the Soviet Union from expanding communism outside its borders.

Explanation/context:

The policy of containment focused on keeping communism and the Soviet Union's influence limited, rather than by trying to confront the Soviet Union directly or eliminate communism completely.  It influenced US foreign policy by prompting intervention in places like Korea to stop the spread of communism.

George F. Kennan recommended the policy of containment which set the tone for US involvement in world relations following World War II.  Kennan was an American diplomat in Moscow after World War II.  In 1946, he sent what became known as "the long telegram" of his advice about what the USA needed to do about the Union of Soviet Socialist Republics (USSR).

In those days, everyone feared an ultimate confrontation between the USA and the USSR -- that the Cold War would someday explode into a massive heated conflict between the superpowers.  Kennan, in Moscow, had much foresight to see the internal problems the USSR had.  He advised not pushing the conflict too much, but instead just try to "contain" the Soviet Union and wait for their system to collapse under the weight of its own problems.  Kennan was right.  It took almost 50 years, but eventually the communist system in the USSR fell apart.  [The Union of Soviet Socialist Republics came to an end in 1991.]