Question

How did lessons learned from World War II influence President Truman’s decision to send troops to aid South Korea after North Korea invaded?.

Answer 1

The West appeased Hitler prior to WWII by granting him concessions in the expectation that he would be satisfied with his territorial gains. In Korea Truman followed the "Truman Doctrine" which was that communism would be allowed to remain in the Nations in which it had already taken root, but would go no further. 

Of the other answers Congress did not declare war, the Korean war was a UN effort with troops from many other nations. 

Answer 2

The answer is because He knew appeasement, or giving in to an aggressor, was not the solution.