I disagree. Russia actually has the logistics and lift capability (well, somewhat) to do something like that if they had total surprise and they weren’t planning to hold it for very long. Flat out, China doesn’t…nor does Korea (either of them) or Japan. They simply don’t have the assets for it.
I agree that Canada doing something like that opens them up for a counter invasion, so not something that’s very likely. Then again, it’s pretty freaking unlikely Russia would invade Alaska either. 
Okay, so how does the US military get around the Posse Comitatus Act to conduct the operations necessary in a state to repel an invasion?
Congressional authorization is allowed to override the act for one, but what ‘posse comitatus’ actions are you expecting the military to do in preparation for an invasion? Rounding up saboteurs?
The Coast Guard also is not subject to Posse Comitatus and are the only group that is both fully military and fully law enforcement.
The Japanese captured the Aleutian islands in WWII. Couldn’t they drive over on highway 9, for that matter?
AIUI, Posse Comitatus means the military can’t be used for law enforcement. Repelling a foreign military invasion (i.e., Mexican army) is entirely different - indeed, as fundamental a military mission in nature as can possibly be. It has absolutely nothing to do with law enforcement and Posse Comitatus would be irrelevant.
The US Posse Comitatus Act included some exceptions from the beginning and has been amended several times, and there are many situations in which the US military can currently be used for law enforcement, and will now be so used.