Pls help Why did the United States get involved in the Vietnam War?

Define: Domino Theory

Which amendment lowered the voting age? WHY?

1. The United States got involved in the Vietnam conflict keep communism from spreading throughout Southeast Asia.

2. A theory that if One country is taken over by an expansionist especially Communist neighbour party or the like the nearby nations will be taken over 1 after another.

3. The 26th ammendment prohibits the states and the federal government from using age as a reason for denying the right to vote to citizens of the United States who are at least eighteen years old.

Hope this helps