In my novel, Game Misconduct:
Game Misconduct - The home of author Jeff Rose-Martland
I address the issue. Here's the scene:
The US has invaded. American troops are pouring across the border. Much of Canada's military is away on peacekeeping duties, leaving us even more vulnerable. The Government has recalled the reserves and militias, and is now calling for volunteers. Would you take up arms or try to get yourself and your family out of harms way? Serious answers only; Rambo is not a Canadian.
I haven't read the novel, but the link you provided does mention it's a spoof. So I take it it might be intended as a humorous story mocking misunderstandings between countries caused by the media leading to war.
If it is in fact a tongue-in-cheek novel, then I can't fully imagine its full plot.
However, if it's intended as a more serious novel, many obstacles would stand in the way of a US invasion, which the author would need to break down before the audience could accept. Among the obstacles:
1. Language and culture. Most Canadians and Americans speak the same language, share the same Christian Faith, and share many other cultural similarities owing to the common British roots. This would thus make it extremely difficult for any US administration to demonize Canadians in the face of English-language news media fully comprehensible to the vast majority of Americans. Combine that with modern technology such as the internet, and this makes it extremely easy for Canadians and Americans to exchange ideas with one another at the grassroots level.
Looking at it that way, a civil war between English-Canadians and Quebecers, or a culture war between Anglo-Americans and Hispanics would be more likely than a war between the majority of Americans and Canadians.
2. Travel, trade and intermarriage. As a result of sharing this common language and culture between the majority of Canadians and Americans, travel, trade and intermarriage is much easier too. With so many Americans and Canadians traveling to each others' countries, trading with one another, and marrying one another, it would make it difficult for the US to attack Canada without causing many Americans to arise in Canada's defense.
Churchill said it well at Harvard in 1943 when he said:
"This gift of a common tongue is a priceless inheritance, and it may well some day become the foundation of a common citizenship. I like to think of British and Americans moving about freely over each other's wide estates with hardly a sense of being foreigners to one another."
It's no coincidence that English-speaking countries have much closer ties with one another than with other countries:we can actually understand each other.
I'm not saying a war between the US and Canada would not be possible, but it would require extreme circumstances, and any serious novel depicting a US invasion of Canada would need to develop such a circumstance to make it credible. Add to that that a US invasion of Canada would not only require a special reason for doing so, but it would also have to be accepted that, owing to the degree of interaction between our cultures, it would be likely that some Americans could end up supporting the Canadian side and vice versa depending on the reasons for the war, be they ideological or otherwise. This means any war between the US and Canada is likely to break out into civil war on both sides as each side takes its ideological stance. Likely the only places where the battle lines would be more clear, at least relatively speaking would be around the province of Quebec and perhaps certain parts of the US primarily very close to the US-Mexico border.
Our two cultures are just too integrated for any US invasion of Canada to be so clean-cut.
Now as to what I'd do if the US ever invaded Canada? Well, that would depend on which side is fighting for a just cause of course. A good example is Hasegawa Teru, a Japanese woman who'd helped the Chinese side against Japanese occupation owing to her strong sense of patriotism which did not allow her to tolerate such attrocities on the part of her countrymen. We could also take the example of those Chinese who'd welcomed her among their ranks as a friend, compatriot and comrade in spite of her being Japanese. They recognized she was a good Japanese, and so embraced her with open arms. She'd even helped to set up an organization of Japanese living n China who opposed Japanese aggression.
At the end of the day, a good man does not fight for hs country; he fights for justice. If that is compatible with fighting for his country, then on the surface it might appear that he's fighting for his country. But the second his country's no longer fighting for justice, that's when the distinction becomes clear. Those fighting for justice will immediately switch allegiances, whereas those fighting for country right or wrong will keep fighting blindly for the country.