Are we brainwashed? Yes, but not in the way most people think.
It's a brainwashing for the cult that is the Marine Corps. When I was in Recruit Training and throughout most of my time in, it was also a "Corps first" mentality. Even over country it seemed.
And it plays even when we get out. There is an instant connection between Marines, even former Marines. A bond if you will. Marines tend to look out for one another, even if they never served together. This isn't always the case, but most times it is, especially among Marines of the same MOS and wars.
I would be willing to bet if there was some sort of defection of the Marine Corps against the rest of the United States, that most Marines would side with the Corps. Just a hunch.