After the American Revolutionary War ended in 1783, a large group of loyalists to the British Crown (Tories) immigrated into French-speaking Canada from the new country of America.
In 1791, with this large English-speaking aggregation now living in French-speaking Canada, the British Parliament made its move. In a Constitutional Act, it split its Province of Quebec into Upper and Lower Canada, and each entity would then have its own representative government and its own preferred language.
In the early 19th century tensions peaked between America and the British Empire over maritime power abuses, and the War of 1812 was the result. Upper Canada and Lower Canada were quickly embroiled in that conflict as America launched invasions against British forces on the Canadian frontier, and vice versa.
In the end there were no real land gains or losses on either side, but some historians believe that the War of 1812 shifted American migration desires and interests from Canada, west into the Great Lakes area, and beyond.
After the war, waves of English-speaking immigrants from the British Isles continued to settle in this new frontier, mostly in Upper Canada. By mid-century, that great migration had brought nearly 750,000 newcomers into the area.
After decades of political and social disputes between the English and French factions, the British Parliament made its move. The Act of Union (1840) essentially merged both Upper and Lower Canada into a single centralized governmental entity, appropriately named - the Province of Canada.