Why did the US become invoved in Vietnam war and why did they subsequently withdraw from armed conflict in Vietnam

https://www.dropbox.com/s/f7gkqahfv7xt5b3/Why%20did%20the%20United%20States%20become%20involved%20in%20armed%20conflict%20in%20Vietnam.pdf?dl=0

https://www.dropbox.com/s/o2g7bjflrq7g2fm/Why%20did%20the%20US%20withdraw%20from%20armed%20conflict%20in%20Vietnam.pdf?dl=0