I wouldn't say, "cocky." The US was absolutely brutalizing the country. More bombs were dropped on Vietnam than were used in the entirety of WW2. Our willingness to use chemical weapons- Agent Orange- on the Vietnamese populous speaks to how far we were willing to go.

The Vietnamese government claims- and this is probably the most liberal projection- that 3.1 million people died in total, including all sides, civilian and soldier alike, died in the war. More detail oriented research points toward a smaller, yet equally sobering 882,000 Vietnamese deaths, and about 58,000 US casualties.

(http://faculty.washington.edu/charles/new%20PUBS/A77.pdf)

The issue was that the US had an appalling poor grasp of Vietnam, the region's politics, and what concerned the average individual. Whether you agree with the man's politics or not, McNamara (secretary of defense under Kennedy and LBJ) actually speaks to this in the documentary, "Fog of War" which I think anyone of voting age in the US should watch. You can find it on youtube.

Regardless, what ended up happening was that Vietnam was fought like WW2, except the US had no concept of how willing the Vietnamese were in terms of the ability to fight, and why they were really there.

Even at the propaganda level where you saw a massive mobilization to mind US public opinion of WW2, you see much less surrounding Vietnam, though that may have more to do with how unpopular- and particularly how loud the anti-war segment- the war got.

So I wouldn't exactly use the term, "cocky." Unprepared? Definitely.