Or vice versa.
Who wins? Who loses? What does the US do? What happens if the US tries to remain neutral? How will the conflict be fought then?
Neither country has the military capability to project power onto the other in significant amounts. Neither country has a strategic air force. The Canadian army and air force are more modern and have substantial combat experience, but wouldn’t be able to land a force in Mexico large enough to make much of an incursion. I suppose the two countries’ navies could shoot each other up, but that’s about it. It would be a totally pointless war, unless the two countries were fighting over some overseas possession like a little Atlantic island or something.
Maybe there is no US any more, and the war is about how much of the former US territory will be absorbed by Canada or Mexico.
Canada is a member of NATO, whereas Mexico isn’t. If attacked by a third-party state, a NATO member is able to invoke the mutual defence clause of Article 5 of the NATO Charter.
Can’t fathom why either would attack each other, though, although if something did kick off Uncle Sam would be more-or-less guaranteed to get involved and take the side of Canada, barring something completely egregious. More shared history and culture than it has with Mexico.
Canada could probably whip up a nuclear weapon a lot faster than Mexico could; at that point they win.
When I was a kid I had a dream that the playground of my elementary school was the Canada / Mexico boarder (I used to live in the interior of BC, so Mexico gained a lot of ground). It was riddled with landmines so I was flying my friends to safety. And by flying, I don’t mean with a plane. I could just fly.
So if I am in any way psychic then Mexico is going to kick some ass.
I’d imagine that their response would be something along the lines of, “If you two don’t stop that right this minute I’m turning this car around and going straight home, do you understand me?”
The US breaks up–different parts taking different sides–eventually losing territory to both Canada and Mexico.
I would worry about the US being caught in a pincer movement. No one likes to defend a two-front war.
The war has already started, and Mexico is winning - they are killing our tourists one at a time.
Why anyone would travel to that viper’s nest for a vacation baffles me.
Why? Did they make the sin of putting Canadian Bacon in a Burrito?
Just split it at the Mason-Dixon line.
The USA would come to aid of their NATO ally and kick some serious Mexican butt.
Now, this assumes in your scenario that the USA still exists. If some weirdness happens and we disappear as a nation and Canada invades southward and Mexico invades northward and they meet somewhere like Nebraska, then all bets are off.
In that case, I would still pick Canada.
Has anyone considered that approximately 50% of Mexico’s population belongs to some kind of paramilitary gang and has an uzi?
Edit: the Canadians have Mounties.
The Mexican military would march on foot to Canada though the US, skillfully leaping the American borders in their determination to win the conflict. Many Mexican American supporters will join in the march, young and old, rich and poor, the numbers will swell- as this is one of the only times Mexicans from both sides of the border will see a reason to unify.
Canada would be unable to nuke the Mexican military, because of its close proximity to non-Mexican Americans in the US. And because only civilians are left in Mexico-- Canada wii fear international reaction to nuking them.
Along the several million man and woman march to Canada- much race mixing will occur, because American women will be charming the pants off of Mexican men, and visa versa. There will be great memories made around the campfire–for manana we may die.
Before we get to Canada, we will be so mixed with Americans- that Canada will call off the conflict and instead invite us to sing, dance and party with them.
American and Canadian babies will be bilingual, and more attractive for decades to come.
Scenario #1. Mexico invades Canada. Mexican ground forces land at Sept-Iles and attempt to march inland and take Ottawa. Unfortunately they neglected to bring a French interpreter and cannot ask for directions. Mistakenly, they advance northwest and are overtaken by the canadian winter. Since the mexican armed forces are issued neither long underwear nor insulated socks the entire force freezes to death.
Scenario #2. Canada invades Mexico. Landing at Puerto Vallarta, the Canadian forces bravely march inland, intent on taking Mexico City. Unfortunately the supplies of Molson run short and the troops are forced to drink water. Upon reaching Guadalajara, the entire force succombs to Montezuma’s Revenge.
Meanwhile back in the states, David Letterman’s Neilsen ratings go through roof with his new line of tasteless Chicano-vs-Canuk jokes.
If such a war happened, and Canada and Mexico were both reduced to shambles, then the WORLD wins.
My response to the thread title was, “Not yet, you fools! Did you not receive the schedule?”
Or Corona. Same thing, really. 
Obviously, in such a war the logistics of getting over the US is going to be the sticking point. However, it seems like it wouldn’t be too hard for Canadian’s to sneak into Mexico in mass under the guise of ‘vacation’. A sneak attack would be lauched by loading up 30 West Jets with troops disguised as tourists. Once they are in place a secret signal will be issued and Puerto Vallarta will quickly fall as Joint Task Force 2 leads the charge of thousands of Canadian troops taking over the airport and key areas. 60 CF-18’s fly in and secure the skies. Ships out to sea land and unload some heavy equipment.
And really, once we’ve captured a resort town or two, we’re done.