1.Dubious means doubtful and a crusade is a holy war.
He is saying that Vietnam did not have a lot of support. I think this will be a common theme throughout the war.

2. The overall theme was death following vietnam. Change was another theme as well.

3. The incident that brought us into Vietnam has always been contreversial.
It allowed President Johnson to defend American ships from Vietnamese attacks. Some say that Johnson made a mountain out of a mole hill to send troops to vietnam. It relates because Johnson didn't have very much support to do this and the war wasn't very supported either.