The 1763 treaty ending the French and Indian War saw a redistribution of North America among the Powers of Europe: In order to get British-occupied Cuba back, France's ally Spain ceded Florida to Great Britain. By way of compensation, the real loser of the war, France, ceded its Louisiana colony to Spain.
Great Britain had big plans for its newly-acquired territories, and wasted no time in putting them into action: George III's Proclamation of 1763 set up the colonies of Quebec, West Florida, East Florida, and Grenada. Later history would guarantee that only Grenada would be a money-maker for its backers.
West Florida included the stretch of the Gulf Coast between the Mississippi River and the Chattahoochee and Apalachicola Rivers. Settlement was concentrated around the old French settlements of Biloxi, Mobile, Baton Rouge, and Natchez, as well as the capital, the former Spanish fortress at Pensacola. New Orleans, being on the correct side of the Mississippi but the wrong side of Lake Ponchartrain, was part of Spanish Louisiana.
The Proclamation set the colony's northern boundary as a line extending due east of the confluence of the Mississippi and Yazoo Rivers to the Chatahoochee. Since the Chickasaw and Creek Indians were still very strong in the region, this boundary meant little and settlement never extended very far from the coast.
In order to stifle competition, the land developers behind the Proclamation made sure that George also prohibited the expansion of the existing British colonies in North America west of the Appalachians. That would cause a bit of trouble later, being one of the principal irritants that caused those colonists to rebel against Great Britain in 1775. The Revolutionary War saw West Florida as a stronghold of Loyalism. When France entered the war in 1779, its ally Spain entered as well. The Spanish governor of Louisiana, Bernardo de Galvez, quickly captured Baton Rouge, Natchez, Mobile, and Biloxi, but a hurricane damaged his fleet in 1780, and the capture of Pensacola required another two years, reinforcements from Havana, a four-month seige, and a lucky explosion of a powder magazine. (Lucky for Galvez, that is; the explosion killed 100 British soldiers). Pensacola finally surrendered on May 10, 1781, five months before the surrender at Yorktown. The 1795 Pinckney Treaty gave West Florida north of the 31st parallel (including Natchez) to the new United States. This line can now be seen as part of the boundary between Alabama and Florida, and part of the boundary between Mississippi and Louisiana. The inhabited southern part was governed by the Spanish, although most of the settlers were Americans.
Spain was the weakest sister in the European coalition against Revolutionary France, and in 1800 they quit the war, accepting Napoleon Bonaparte's terms in the the Treaty of San Ildefonso, which allowed French troops to occupy Spain, and transferred Louisiana back to the French. Napoleon, having enough to do fighting off the rest of Europe, turned around and sold Lousiana to the United States in 1803. Soon after, the Spanish people rebelled against Napoleon's occupying troops. This caused Napoleon to put his brother Joseph on the throne of Spain. The British saw an opportunity and landed in Portugal in 1808, touching of the Peninsular War.
The loss of Spanish control over Spain made the governance of Spain's colonies in the New World that much harder, and rebellions sprung up all over Latin America. In 1810, a group of American settlers declared West Florida independent from Spain and set up a republic. A blue flag with a white star flew over the capital, Baton Rouge, for a few weeks. Then, United States President James Madison reinterpreted the Louisiana Purchase to include West Florida, and sent troops to annex it. This alarmed the Spanish rebels in Texas so much they decided not to ask the United States for military help, and gave up their rebellion.
British naval supremacy during the War of 1812 meant that West Florida was easily re-occupied by the British, but they returned the territory to the US with the Treaty of Ghent. West Florida became a permanent part of the United States, now part of the states of Louisiana, Mississippi, Alabama, and Florida (the rest of which was acquired in 1819).