Eh, I'll give it a shot.
First, before it can have its civil war, Mexico has a mostly peaceful military coup that ousts Díaz, and manages to win much of the populace over with promises of... erm... of not being Díaz, I suppose. This launches an era of prosperity, as the new president/emperor/what have you is actually a capable ruler.
When the Zimmerman telegram comes, they see it as an opportunity to reclaim parts of the US that were once Mexican and declare war on the US. The Mexicans manage some quick victories and take some territory but suffers badly towards the end of the war. Ante-bellum borders are agreed upon.
Meanwhile, the US declines to enter WW1; the second battle of the Marne turns into a bloodbath; the Germans are defeated but only just. The war turns into another stalemate and the conscription crisis leads to widespread revolts The workers are marginalized as more and more of the factories there are made to fuel the British war effort. Socialism becomes very, very popular in western Canada. Quebec declares independence from Britain and asks to be reunited with France, but the rebellion is crushed by Britain. France is upset that Britain would divert troops from the European theater at a critical stage to protect its own interests abroad; this is the beginning of a souring in anglo-french relations.
France suffers widespread revolts and a failed coup in 1922 as a result of the war; many factions were at work in the coup, but the communists were the ones blamed. There is a mass exodus of communist sympathizers after the war, who mainly travel to Quebec.
At the London Peace Conference (Paris being deemed "too unstable" due to the revolts at the time), Britain is wary of France over the Quebec issue. It makes an effort to improve Anglo-German relations by undermining France offering a much better deal for Germany; the US uses this opportunity to negotiate to make Mexico pay reparations for their own war.
After the war, increasing mistreatment of workers and the influx of immigrants (particularly asians) leads to widespread protests. Socialist uprisings result in a coup and Canada declares itself independent from Britain in 1927.
This triggers a massive red scare in the US. Fears of both external and internal conflict fuel mass hysteria and riots. The fears of factory strikes and turmoil leads to a collapse in the stock market in 1928, an election year, which leads to actual radical uprisings. In the aftermath, a far right-wing president is elected; he soon assumes emergency powers to deal with the situation. What follows is a campaign of repression and severe curtailing of freedoms, which is lauded as a "victory against anarchism and radicalism."
The US now has a destabilizing element to the north, a hostile enemy to the south, and perceived radical elements within. They attempt to seed a rebellion in Mexico, which is botched and leads to the US declaring war pre-emptively in 1933.
Much of the international community condemns this as an act of unprovoked aggression; Canada, after lengthy discussions with its allies, decides to show the world its goolies by declaring war on the US; Russia and France soon follow. Britain, sensing that the Americans are in deep shit, decides not to intervene on their behalf.
The Canadian invasion of America goes about as well as expected, even with massive foreign help. The US eventually finishes up in Mexico and establishes it as a puppet state; after a game of tug-of-war in the Great Lakes region and immense losses on both sides, an armistice is declared in 1935. The "Second world war" as Americans call it (the rest of the world calls it the War of American Aggression) turns out to be much smaller than the first.
The United States is now in a unique position. It is so powerful, yet in such a relatively remote geographic position, that it can't be seriously threatened.
In the next few years, most of the world tries to prop up Canada as a powerful counterbalance to the United States. With The US having become xenophobic, anti-immigration sentiment begins to crop up, and immigrants typically headed for the US instead go to Canada. Russia and France both send massive amounts of money and arms to bolster its defenses. Canada's population, military, and economy all boom fantastically.
The USA carries itself with an air of arrogance during this time, but the war had seriously damaged it. Hundreds of thousands of men had perished, by far the largest number of Americans in any war in its history. The government tightened its grip on the populace, which was increasingly anti-establishment, and as much as it likes to bare its teeth at its northern neighbor, it doesn't dare attack.
Britain, however, is on the warpath. There was considerable turmoil over seemingly betraying their American allies, and the ruling party's perceived soft stance towards the French and communism struck a sour note among the population. Britain and the US begin to send out diplomatic feelers to China and Japan, respectively, using the common threat of the red bogeyman.
By 1943, cross-channel tensions have reached an all-time high. Denmark, an ostensibly neutral country, is allowing French warships into the Baltic Sea, which essentially allows them to blockade most of Germany, which has become more or less Britain's pet. There was little recourse other than As a result, Britain threatens Denmark to refuse the French access. Denmark tells France and the resulting international incident triggers the real second world war. Britain, Germany, USA, and Japan versus France, Italy, Russia, and Canada. Britain believes that it can win such a war quickly.
As it turns out, they were mistaken. The United States was on the brink of serious collapse. The population had vivid memories of the bloody war with Canada eight years prior. Canada makes significant gains until the US pushes them back. After many, many casualties, the American people simply cannot take it any more. Massive protests erupt. The army is called in to suppress the rebellion, which sparks even more massive protests. Army units defect and the US collapses into civil war, with Canada there to sweep up the remains.
With its most important ally now in turmoil, Britain's axis is completely ruined. A major rundown of the aftermath of the war:
- Several American states wish to be incorporated into Canada: New Hampshire, Vermont, Maine, Alaska and Washington,
- The British empire is dismantled. Scotland is made independent, Northern Ireland is given to the Republic of Ireland, and Britain's colonies are either granted independence or given to one of the victorious powers,
- The war ends with the first hostile use of a nuclear weapon against Sapporo and Sendai by the USSR,
- Britain and the USA are forbidden from expanding their military, from seeking colonial expansionism, and from obtaining nuclear weapons.
The French are disillusioned with some of the socialist government's practices and its conduct during the war. After the war, France, for the first time since before WW1, elects a non-socialist government. This is the first domino in a long string which ultimately leads to the famous Franco-Soviet Split of the late 1950s. Canada joins France, while most of the European nations and soviet satellites, obviously, join Russia. China is split between North and South, Germany between East and West, the former Raj is split up, and Japan becomes lodged firmly in Russia's sphere.
France, however, is plagued by revolts in its African and Indochinese colonies; the government mismanages the affair and ends up thoroughly humiliated, and all of a sudden Canada finds itself as the leader of a sizeable chunk of the world. In the coming years, the United States would be forbidden from producing nuclear weapons, while Canada develops its own native nuclear capability, with French help of course. Canada has officially risen to become, to use an antiquated term, a Great Power.
GOD DAMMIT BRAIN I NEEDED THAT TIME TO SLEEP!
Well, anyways, I don't have any idea how borderline (or past-borderline)-ASB this timeline is; I stopped caring about research around 1940.