Mostly the fact that we weren't equipped to fight against North Vietnam. Their military was very well trained and equipped. If I remember correctly, we actually withdrew from the war because we couldn't fight off the Northern soldiers.
There was one incident when we were going around NV to get to SV with our navy, and NV's navy attacked our ships. I think that probably contributed because we would've had trouble getting our Navy to SV.
But the first statement was the most difficult part, from what my lessons said at least.
