Answer:
The background of the Cold War
Step-by-step explanation:
Vietnam was important for the US policy of containment because their main goal (or rather irrational fear) was to stop communism from spreading.
The US got involved military because they couldn´t imagine that a small peasant country in Asia could stand up to the world´s greatest power.
LBJ, Nixon and everybody else in the White House underestimated the determination of the Vietnamese people.
They were not, it was actually contraproductive because it turned apolitical rural people into convinced communist fighters.