France effectively lost New France after the British victory at the Battle of Quebec in 1759, but prolonged the Seven Years War until 1763 in the hopes of acquiring enemy British colonial possessions in the conflict to be used as leverage or traded in the peace negotiations. From the British perspective, the lands east of the Mississippi were to be designated as "Indian territory" according to the 1763 Royal Proclamation that forbade white settlement west of the Appalachians. Colonial American settlers largely disregarded this and continued migrating westward.

In 1762, under the secret Treaty of Fontainebleau, France agreed to cede Louisiana to Spain. The Spanish were absentee landlords and, outside of the environs of New Orleans, the territory remained largely undeveloped. Spain haphazardly reasserted its claim to the region in the late 1790's when it briefly revoked the Americans' previous rights granted under Pinckney's Treaty (1795) to use the port of New Orleans and to navigate the Mississippi.

These rights were soon restored, only to see Spain cede Louisiana back to Napoleonic France under the secret Third Treaty of San Ildefenso in 1800. France would not formally take possession of Louisiana until Nov. 30, 1803 -- a mere three weeks before it ceded the territory to the U.S. on Dec. 20, 1803.

Napoleon was preparing for new campaigns in Europe, with the 200,000-strong Armee de l'Angleterre assembling in northern France and poised to invade Great Britain. The sale of Louisiana to the U.S. for $15M was intended to help finance the invasion. Nelson's victory at Trafalgar in 1805 put an end to this threat, so Napoleon redirected these resources eastward to continental Europe, leading to his monumental victory at Austerlitz that year.