This movie is the based-on-true story of the first detachment of American soldiers to Afghanistan after 9/11: twelve (see title) Green Berets tasked with helping the native anti-Taliban fighters take the strategic town of Mazar-i-Sharif. The big stars are Thor himself, Chris Hemsworth, playing Captain Mitch, the leader. He is assisted by Michael Shannon and Michael Peña.
The movie unashamedly admires the soldiers' devotion to their duty, even when it involves them leaving their loving families to go on a mission where the odds of them returning in one piece aren't very good. There's nary a hint of the usual Hollywood anti-Americanism here. The movie also does an unusually decent job of setting up the details of battles, what the stakes are, the geography of the area, and how things unfold.
There are a lot of very boilerplate war-movie tropes, too. For good reason, of course, but I found myself saying "Yeah, saw that coming" at a number of points.
It was especially appropriate to watch this movie in the context of current events, the President wanting to negotiate with the same guys we were trying so hard to kill back then, with the probable outcome of them taking over the country once again.
As always, the History vs Hollywood site is your go-to for how sorta-true this movie is. Not too bad, as it turns out.