Truman was the one who adopted a policy of containment. This (arguably) aggressive approach was what influenced America to get into wars such as the Korean War and other smaller conflicts. Meanwhile, Eisenhower was a bit less aggressive. Although his Secretary of State Dulles preached Brinksmanship and was very anti-Soviet, Eisenhower himself was somewhat against this, as shown when he did nothing to prevent the Hungarian Revolt. He also sought to increase U.S-Soviet Relations in the Spirit of Geneva, but the U-2 Incident pretty much erased most of his progress.
The United States persecuted U.S. citizens.
The United States began to build up its military and to stockpile weapons.
After fighting the war in Korea, the US had learned that communism would spread. In order to keep with their policy of containment, they had to rush in and stop Viet Cong from spreading communism (much like the domino theory that the US had). Had Viet Cong not wanted to bring about communism, there would be no need for invasion, and the domino theory would have yet to be proven.
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