The united states entered the conflict on the korean peninsula in the name of

The Cold War Policy, otherwise known as "The Containment Policy"
It was designed to prevent the further influence and spread of communism. 
The United States had entered the Korean conflict to provide support to South Korea, as the North had declared war on the South in attempt to convert the entire Korean peninsula into a fully communist state.

The war has not been officially ended, and the North is still resented toward the U.S. to this day...

Hope this helped!

:V

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