The Vietnam War – Sourcework Questions: Why did the US get involved in Vietnam?

jfkSourcework Questions: Why did the US get involved in Vietnam?
Several sourcework questions, culminating in:
Here are three reasons why the US may have got involved in Vietnam: which do you agree with most, and why?
i. Because they wanted to protect the world from communist dictatorship 
ii. Because they were paranoid about the communist threat
iii. Because they were aggressively trying to dominate the civilised world