Don't get me wrong, the United States could probably still do it, but it wouldn't be a simple walk in the park either.
You're joking eh?
Vietnam, Iraq, Afghanistan... Did you forget the lessons taught here (well obviously, if you remembered what happened in Vietnam, Iraq would never have happened).
When a population is committed to opposing an invading force the invasion will never been finished. The Troubles didn't end through the martial might of the British, it ended because the cause of the conflict was resolved. For 48 years the British tried to use force to solve the issue, in the end it was diplomacy that bought the conflict to an end. When a population has cause to fight, they will continue to fight, Sun Tzu called this "the moral law", whichever of the sides the people believed to be more right was imbued with the moral law.
The reason the US has been able to hold onto states like the Dominican Republic and Grenada is the fact the majority of the people accepted that the US was the better side in the conflict. Vietnam and Iraq on the other hand, the US was seen as the worse side, supporting corrupt governments and ideals.
If Canada was resolved to fight a US invasion dont kid yourself, you'll never hold Canada.