The Senate approved the treaty finalizing the Louisiana Purchase, an acquisition Napoleon Bonaparte agreed to after his ambitions in the Americas collapsed following the loss of Haiti to revolution and renewed war with Britain. The deal roughly doubled the size of the United States overnight, opening the trans-Mississippi west to American expansion and setting the stage for decades of westward settlement, conflict with Native nations, and the spread of slavery into new territories.
Napoleon had originally intended Louisiana as the breadbasket for a revived French empire in the Caribbean, anchored by the sugar colony of Saint-Domingue. But a slave rebellion led by Toussaint Louverture, followed by a devastating yellow fever epidemic that killed thousands of French troops sent to suppress it, wrecked that plan. Facing renewed war with Britain and needing cash, Napoleon abruptly offered to sell the entire territory rather than the New Orleans access that American negotiators Robert Livingston and James Monroe had originally been sent to secure.
The price came to less than three cents an acre for what would become all or part of fifteen future states. President Thomas Jefferson, a strict constructionist who had doubts about his own constitutional authority to acquire foreign territory by treaty, nonetheless pushed hard for ratification, reasoning that the opportunity was too consequential to squander on legal technicalities.
The Senate's ratification set off a cascade of consequences still felt today: the Lewis and Clark expedition, the forced displacement of dozens of Native nations, and the extension of slave territory that would fuel sectional conflict for the next six decades, ultimately contributing to the Civil War.