Enough people seem to be responding to my "socialist revolution" scenario that I'm thinking I'll try to work out a quick TL:
1880s: Canada's "Liberal party" wins a number of electoral victories, bringing in a government that is pro-free-trade and opens up trade restrictions with the US
1890s: the Liberal party has begun to be dominated by Canada's few industrialists which enact policies to keep wages low and open borders to European immigrant workers. However, its free trade policies keep it supported by Canada's farmers. The Conservative party remains the official opposition and is dominated by pro-British reactionaries, but a series of hapless leaders prevent them from defeating the Liberals
1900s: industry is beginning to flock to Canada due to low wages and government subsidies, and many Canadian industries now export to the US. American leaders begin to call for the reinstatement of tariffs, but the Canadian industrialists running the Liberal party make sure to "donate" shares in their large companies to American presidential candidates. The Canadian Labour Party is founded by moderate labour leaders while the more radical ones go one to organize the underground Socialist Party. Labour-led strikes and protests are brutally repressed by police.
1910s: Toronto, Hamilton, Windsor, and St. Catherines have become the industrial centres of Ontario, with industrial output that rivals that of Detroit. Montreal, however is still the largest city (by far), the headquarters of the railroads and the banks, and the centre of political dissent where the Francophone working class is dominated by the Anglophone industrialist class. Winnipeg has grown into a small regional centre, and Halifax and Vancouver also have limited amounts of industry centred around their ports, but most of Canada outside of Ontario and Quebec is still largely rural. The Labour Party begins to gain votes every election until they reach 25% of the popular vote, while the Liberal Party sheds votes. The production of war material for WW1 leads to an economic boom and even further industrialization.
1921: The Liberal party is beset by a scandal, and they dip below 40% of the popular vote for the first time in 30 years. The 1921 elections see the Liberals capturing 37% of the vote and 38% of the seats in Parliament, Labour capturing 43% of the vote and 47% of the seats in Parliament, while the Conservative party is able to capture 15% of the seats with the remaining 20% of the vote. However, before the the Governor General calls upon the Labour party to form the government, the Liberals and Conservatives approach the Governor-General asking to form a coalition government headed by the Liberal leader. Labour leaders call "foul!" arguing that the Governor-General is loyal to the Industrialists and asking that they be given a chance to govern. However, the leader of the Labour party decides that the only constitutional method of challenging the Governor-General's decision is to go above the Governor-General to the King himself. A number of representatives of the Labour party depart for England to meet with the King.
1922: The result of the 1921 elections is met with unrest, particularly in Ontario and Quebec, where the Labour party received a majority of the popular vote. The Labour party, now anxious to be seen as a "legitimate" political party, refuses to associate itself with the protests, and membership in the underground Socialist party continues to grow, especially in Montreal. The protests turn into riots which turn into a revolution, and the Socialists take control of Montreal and Hamilton, with battles taking place in Toronto, Windsor, and Quebec City. Ottawa is almost surrounded, and the governor-general and the leaders of the Liberal and Conservative parties soon flee West by train to Winnipeg.
1923: The revolutionaries, arriving in Ottawa, call upon the Labour Party to form the new government. However, this government is led by the radical wing of the Labour Party, as the moderates are still in England (they have feared returning to Canada due to unrest). Meanwhile, the government in Winnipeg has called upon American assistance as new riots are growing in Winnipeg itself, and American troops first enter Canadian soil in Manitoba. The Socialist government in Ottawa objects to the presence of American troops in Canada and begins to raise its own revolutionary armies.
Later that year, the first direct confrontations are made between American and Revolutionary forces. The Americans succeed at taking control of Windsor and St. Catherines, but are stopped by the onset of winter.
1924: A British fleet arrives in Halifax, where the police are trying to maintain control in the name of the government in Winnipeg. The British are soon in control of all of the Maritime provinces, while the Americans are still supporting the Liberal-Conservative government in the West. The American troops are now in control of most of Southwestern Ontario but are stopped in their advance down the Richelieu River towards Montreal.
1925: The British have taken control of the Gaspé penninsula, but are having difficulty occupying it, despite the largely conservative, rural population. The problem is that the Francophone population objects to a British occupying force, so Britain calls in their ally France to aid with the occupation.
By the end of the year, the Americans control all of Ontario West of Kingston, and the Eastern Townships of Quebec, the British control the Maritimes, and the French have advanced to Quebec City. Montreal and Ottawa are the only cities still controlled by the revolutionaries, and the remaining Labour Party MPs in Ottawa (who have lost control of the Socialist-dominated government over the last few years) defect to join their colleagues who are with the British in Halifax. Soon talks of peace and rebuilding have begun.
1926: Through the peace talks, it is becoming clear that Canada can no longer be a united country. Having had a government Winnipeg for years, the Western provinces are no longer willing to be ruled from Ottawa, and a six-province Western Confederation is founded consisting of British Columbia, Alberta, Saskatchewan, Manitoba, Northwestern Ontario (renamed Superior), and Arctica (a province formed from a merger of the Northwest Territories, the Yukon, and the Ungava region of Quebec.). The Western Confederation is the part of Canada that remains a Dominion, and is the legal successor of pre-revolutionary Canada.
The Maritime Provinces have been happy under British occupation, and they choose to return to the status of a British colony, and are soon merged with Newfoundland. Eventually, they are renamed the Eastern Confederation and become a Dominion of their own, but remain poor compared to the Western Confederation, and dependent on the British military presence.
Southern Ontario and Quebec are handed over to British and French occupation, respectively. The Americans are already unhappy with the recolonization of the Maritimes, so they demand that Ontario and Quebec be reorganized as Republics. The Labour Party does go on to win the Presidential elections in both Ontario and Quebec. Once the occupation forces pull out in the 1930s, Ontario goes on to join the USA, and Quebec charts its own path as a Republic loosely allied with France.