The Truman Doctrine said that the United States would give aid to any country that was faced with being taken over by a communist insurgency or by communist attackers from the outside. This was how the US first got involved in Vietnam.
After WWII, the French in Vietnam faced an insurgency that was partly anti-colonial, but was also led by communists. This could be seen as an example of "attempted subjugation by armed minorities." The French were defeated and the country was dividied into North and South Vietnam. Because the US felt that South Vietnam was faced with being taken over by this communist insurgency, it started to help the South to try to resist this insurgency. This is in keeping with the Truman Doctrine.