My BJJ training partner for several years (late 1990s) was a former Navy SEAL. Dave had served in Bosnia and was finishing up his undergraduate degree at Oregon. When we started training together he was pretty green and I usually had the upper hand. Physically we were pretty even -- he's about 5"10 and 190 lbs, so a few inches shorter and a bit bulkier than I am -- but my technique was superior. By the time we stopped training together he was a purple belt under Megaton Dias and kicked my butt regularly. The thing I remember about Dave is that he would never quit. A few times I choked him out completely (eyes rolled back, drooling, even memory loss) because he wouldn't tap.
He also never got tired, so after my technical advantage went away I always knew things would go bad for me if we rolled long enough -- he'd just wear me down! In peak condition we'd sometimes go 10 or even 15 minutes before one of us could finish the other. One time we rolled so long I got flat out exhausted and actually tapped because I was so tired I couldn't go on. (To be precise I thought I had him in a submission and blew myself up trying to finalize it; when he reversed the position I was so gassed I just gave up.) Dave was outraged that he'd been denied the chance to really finish me -- he wanted me to keep fighting, but I just couldn't go on! I realized at that moment I'd done something no Navy SEAL would ever do: QUIT! We might be on equal terms as athletes but I had nowhere near his mental toughness. (Mental toughness is what always comes to mind when I watch BUDs training videos, which I love.)
My other SEAL data point was an Annapolis grad, a former wrestler who used to come by the judo room at Yale to spar a little bit. He was a good athlete but didn't know much about submission fighting (this was the mid 1990s), so was easy to tap out. I lost count of the number of times I caught him in a guillotine. Many people think SEALs or other military guys know how to fight hand to hand, but that's a myth. They spend almost all their time training with weapons, which makes sense because unarmed combat is pretty rare on the battlefield. These days there might be some MMA technique taught in the military, but I'll take a trained fighter over a Krav Maga guru any day ;-)