Of course the big elephant in the room is the United States, because they would worry that foreign elements (China, Russia, etc) could gain influence in the new independent Quebec and the Americans would see that as a threat (justified) to their security.
So, 10 minutes after Quebec declares sovereignty, the !0th Mountain Division moves north, along with the 82nd Airborne and/or 101st Airmobile Division, plus a lot of other formations. In fact, the US might just say the heck with it take over the whole country.