Franco-American Alliance
France survived the Napoleonic Wars as a powerful nation which could still compete with the power of Britain. Expansion of the USA was causing strife between Britain, which was sending troops into British North America, and the US. The still weak US needed support. It got support from France, which was still a world power. French support kept the US from being invaded, allowing it to remain independent.
Monroe Doctrine and Implications
The American president, Madison Monroe, declare the Monroe Doctrine in 1823. This doctrine decreed that European powers should not interfere with the Americas any further. The Latin American Revolutions that occurred a few years earlier almost wiped out European control of the Americas. However, Mexico remained under Spanish control. This meant that the US could only expand one way: northward. This would mean war with Britain, hence expansion could not occur unless Mexico became independent. The US financed imperial Mexican rebels, who supported the transformation of Mexico into an empire. This would give the US an excuse to invade Mexico, enabling expansion. These rebels would eventually gain their independence in 1832, in the extremely bloody Mexican War of Independence. Instability caused by republican rebels, who were now supported by the US, began. The weak Mexican Empire would never strengthen to be a strong nation.
Settlement of Louisiana and Aftermath
In the lands of the Louisiana Purchase, Native Americans made their home. However, many Americans sought to establish themselves in the land. The American government realized that settlement of the frontier would help to strengthen the nation against British expansionism. In a series of treaties, Native Americans lost their land. The government encouraged natives to immigrate to British North America. Most of them went to Selkirk's land grant, which gained a more native culture, separating it from European Canada. However, instability ensued in the region due to a mostly nomadic populace. The plan to destabilize British North America was partially successful. Selkirk sent in more and more Scottish settlers, causing violence between the natives and Scottish people. In time, the natives were assimilated, but Assiniboia was unstable. This occupied the British military forces as now, the USA could focus on weak Mexico.
Mexican-American War
In 1836, the US invaded Mexico over what it stated was over the Imperial system. In four short years, Mexico City was occupied by American troops. The Mexican-American War was a swift American victory. In the peace treaty, the emperor was forced out of power and the First Mexican Republic was formed. The most major part of the peace treaty was the secession of over half of Mexican land. Land as far south as New Mexico and Baja California was seceded to the US while Texas was given independence as an American-influenced state.