-like megz2410 said "americas involvent grew because of their domino theory",
the americans at the time precieved communism as like a virus of some sort as they were the opposed to whole capitalist ideaology.
-It was said by some famous american politician (rofl can't remeber who) that if they didn't stop communism in Vietnam the spread of communism would be so great that they soon would be fighting communism in san fancisco.
-and yeah it was the Cold War period in which the two major powers CCCP (USSR) and United States of America were passively having battles accross the globe. (Capitalism Vs Communism)
Correct me on these info if you want, i'm just trying to remember stuff i just learnt like 2 months ago
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Good Luck on your Assessment!