The United States essentially took  a “domino-esque” viewpoint upon the Vietnamese and thought that if they stood by whilst the communist Northern Vietnam took control of the rest of the country, then in turn other countries would be overtaken by communists as well. Since the south could not resist the communist push of northern Vietnam by themselves, the United States got involved and aided the rest of the country in terms of resisting communist takeover. So basically the primary reason the Homefront got involved was to keep the spread of communism at bay.