When Uncle Sam sends you somewhere, you go. To paraphrase, you defend against all enemies, foreign and domestic. And if the situation requires it, origin and nature unknown.
I have never served, so I did not understand all of the military jargon. I treated it like British slang. I don’t get British slang either, yet I know there’s people in the world that do. Maybe it was because my spouse is a vet and my son is enlisted, but I cared about the characters. They were relatable to me. (Which also helped with the jargon. Not understanding it better. I’m just used to not understanding jargon.) The only thing I would change is I would have liked a bit more information about the enemy. A little background, perhaps. There are times when not knowing is more terrifying, but there’s also times when knowing scares the daylights out of you.