In 1837, on Earth-599, a constitutional conflict between the governors & legislative councils of Canada on one side and the popularly elected assemblies on the other led to the Canadian Rebellion. The revolt was worst in Lower Canada, where there was an enormous amount of animosity between the British minority, represented by the governor and council, and the French majority & assembly. As well, attempts in the preceding 15 years at merging Upper & Lower Canada had led to much protest in Lower Canada towards Upper Canada. The rebellion in Lower Canada was initially confined to Montreal and environs, but quickly spread to much of the rest of Lower Canada when the rioters began a series of atrocities against supporters of the government, and the troops sent by the government to suppress the revolt responded in kind. Street fighting spread to every major city, and the government troops found that the will of the populace was surprisingly strong, and that their opponents were quite well-armed. The leader of the rebels, along with his advisors, seized Navy Island in the Niagara River, declaring it a provisional government; this led to a strong force of Canadian army troops crossing the river and storming the island, capturing the rebel leader & all his advisors but one. The Canadian troops also burned three American steamers which had been supplying the rebels.
This did not, however, spell the end of the rebellion, and guerrilla warfare broke out; for the next 18 months an insurrection continued. Moreover, strongly armed bands - American sympathizers and Canadian rebels who'd fled into America - raided into Canada from America. Finally, in late 1838, they provoked a heavily-armed response by the Canadian army, who ventured into American territory and searched out and destroyed terrorist camps in upper New York state and in Vermont. The United States, in a bad way because of the Panic of 1837, and already irritated with Canada over the burning of the steamers and because of the Aroostook County War in Maine, was outraged, and President Martin Van Buren ordered an armed American response.
War broke out between the United States and Canada, and the United Kingdom, despite its regrets, had no choice but to aid its Dominion member. After a short-but-vigorous war, in which New York City and Washington were burnt following the defeat of the American army in pitched battle, the United States surrendered, and was once again made a vassal state of the United Kingdom, being forcibly merged with Canada and its name being taken away. From the Inuit camps on Victoria Island to the Rio Grande, there was only one country: Canada.
104 years later, emboldened both by the presence of so many metahumans and paranormals and by its success in almost single-handedly bringing the Second World War to an end, Canada declared independence from the United Kingdom, and managed, despite heavy protest and the threat of armed force, to make it permanent; it was widely rumored that the armed forces of the United Kingdom, along with the metahumans of its Special Operations Executive, knew that if they took the field against the Canadian armies and the metahumans of the Department J, they would lose, and pressured Whitehall to gracefully accede to the Canadian demands, rather than face a humiliating and costly defeat.
This in turn led to many other countries and peoples attempting, and in large part succeeding, in breaking away from both the United Kingdom and the other colonial powers. World-wide chaos and violence ensued, and a group of Canadian paranormals, seeing the unrest that resulted from the end of the War and from the pains of independence, joined together to form the first true superteam, and to enforce not just in Canada but across the world. Ten years later they have had many more successes than failures and have become heroes and inspirations to the world, and although there are other heroes, and even other teams of heroes, they remain the foremost superhero group of Earth-599. They are...the Avengers!
Author's Note: For those who don't know, the 1837 rebellion actually happened. It's just that, here on Earth-Prime, Martin Van Buren ordered no armed response, and the revolt faded away.