The Reason Why American Entered Vietnam War?

The Americans started the war on Vietnam in 1959  they did this because of the Domino Effect-

as it stands Vietnam planned to turn Communist meaning no one really has the right to freedom and whats your's is shared between everyone.
The Americans believed that if Vietnam although a considerable distance from the USA had the power to turn all the surrounding countries communist
this would then knock  surrounding countries into communism like dominoes if one country falls the rest will surely follow suite.
If this were to take place it would make a mockery of Americas treaty of independence and freedom.

The bottom line being they fought an idea that if became a reality there peace treaty would be worthless.