During President Dwight Eisenhower's administration, he came up with the Domino Theory which said that if one nation in the Pacific falls to Communism, so will the rest of Southeast Asia. President John Kennedy agreed with this theory which led to the major actions that he took against the Vietnamese in the early 1960's.
Because the Americans knew that if they entered the war they would need a lot of money, lives would be lost, it was questionable what would happen with the political scene; whether the ruling party would still have the majority vote once it was all over, they would be more susceptible to attack, etc.
There are many factors to consider before deciding on entering a war.
Italy and Japan are the answers
Vote, choose who they marry or even if they get married, choose to have kids or not, learn arithmetic, have a career, learn to read, I could go on.