We thought that Vietnam was a struggle between communists in the north and anti-communists in the south. But it wasn't. It was a civil war, a struggle with decolonization, and a struggle against American efforts to maintain a chosen regime. As Robert McNamara later pointed out, this was a failure of empathy on the part of United States's foreign policy. We failed to empathize with "the enemy" and consequently were unable to address the root causes-- and potential political solutions-- of the conflict.
Thomas Friedman makes a solid case that we are doing the same with ISIS. The basic question of how ISIS has made such rapid gains in the region (or at least they did a few months ago) is perhaps best answered by addressing ideas of nationalism and Sunni regional pride rather than jihadist ideology.
The Shiites in Syria and Iraq, in this analogy, are Diem's regime: brutal, corrupt, and propped up by foreign powers-- Iran for the former, and the US for the latter. No matter how much the US bombed the North, the Southern regime could never stand on its own nor gain much popular support.