Okay, based on the design, the submarine might be a Virginia-class, which means about 130 (give or take) just got killed.

So, that would be a causus belli I'm pretty sure, except they'll gloss over it or cover-it up or some other thing to abrogate general responsibility for the situation getting to this point.

It's the sort of thing that can lead the international community to go "well, I guess it's time for Sentinels," and that never ends well.