No sure I agree with that article. The idea that 'soldiers don't fight for patriotism or grand ideals, they fight for the guys in the trench beside them' is one I've heard many many times before (including in accounts by actual former soldiers) - it's a very common argument. Not having been in a war myself I don't know, but it always sounded entirely plausible.
I also think the constant harping on about the US being founded on 'ideals' is part of the problem. It's not being honest about the real situation.
Also, I kind of think
Is partially correct. Though, he lists the wrong categories. Very significantly, he misses out by far the most important one - 'economic' - in that list, but there are other ways of defining groups that also matter. Ultimately life is a zero-sum struggle between different groups, that's what politics is.