In Canada at this point in time, its almost a non issue in terms of invasion. The only time a non military citizen would have to contemplate defending our country would be if the US was the invading nation and that's never going to happen. If a merger were to happen it will be written on paper in ink and not blood.
Almost any other nation that would attack us would have to defeat not only our military, but the US military as well, as they would not stand idly by and watch a US non friendly nation take over Canada.
The real question would be if a non friendly nation were attacking North America, since they would have to fight 2 if not 3 countries at once and focus on the US to have a chance of winning. Even if the US brought it on themselves, which would be unfortunate, if the attackers were non friendly to Canada, I would fight and defend, because if the US fell, then its basically guaranteed that Canada would be next.
As for invading other nations, or helping to invade other nations, No, as long as they didn't bring the fight to us initially. There are many other ways to force hostile nations to bend to your will without having to send your people to die. These tactic's take more time to create change as compared to quicker physical force, but are much more peaceful overall.