In retrospect, communism in Vietnam was not such a big threat. However, very few people can truly and honestly say they believed that at the time.
During the Cold War, communism truly did seem like a threat. The Soviet Union was clearly trying to expand its influence around the globe. It seemed at least possible that their continued expansion would threaten the existence of the entire free world. It would have been very difficult for any leader in those days to have simply decided that communism was not a serious threat.
The fact that it was not really "half politics" can be seen in the fact that very different presidents all remained dedicated to opposing communism in Vietnam. Every president from Eisenhower through Johnson (liberal or conservative) pledged to keep Vietnam from becoming communist.
Today, we look back and know that Vietnam's fall was no huge deal. But it would have been very hard to realize that when the US was getting involved in that war.