In 1802, President Thomas Jefferson sent emissaries to France to negotiate the purchase of the port of New Orleans from the French Emperor Napoleon. Just two years earlier, Spain had secretly retroceded Louisiana to France. Jefferson feared that France would ignore the American right to navigate the Mississippi River and trade goods at New Orleans. War in Europe and a revolution in Saint-Domingue, present-day Haiti, in which its enslaved people had won independence from French colonization left Napoleon in need of money. He surprised American emissaries agreeing to sell the entirety of the Louisiana Territory for $15 million. The purchase is one of the most significant events in American history and nearly doubled the size of the country.

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After the purchase, American settlers began pouring into the Missouri Territory. Thousands of enslaved African American workers came to the area – either forced to move with their owners or brought there by the domestic slave trade. As these pioneers migrated into the region they continued to claim more lands, displacing Native peoples in Missouri.