The British ceded their claims to territory that later became part of Alabama in the treaty of 1783 ending the American Revolutionary War in which the U.S. won its independence. It was the Spanish claims to lands that later became parts of Florida, Louisiana, Mississippi and Alabama that proved harder to extinguish, so that the European colonial presence in the region could be ended and the stage set for American expansionism.
Spain had been an ally of France and hence hostile to British interests during the U.S. in the war for independence from Britain. But Spanish-held territory reaching into what was to become Alabama was not annexed by the U.S. until 1813, after the greater Mobile Bay area was captured from British invaders and Spanish colonial garrisons in the War of 1812.
Meanwhile, South Carolina had ceded its claims to western lands reaching into the Mississippi and Alabama region in 1787, and Georgia had done the same in 1802. So Congress organized the lands that later became Mississippi and Alabama into the Territory of Mississipi, the western half of which became the State of Mississippi in 1817.
In 1817 Alabama became a territory under its own federal organic act. Having been part of Mississippi during its period of incorporation under the Northwest Ordinance model for territorial political status resolution, Alabama was fast tracked and became a the 22nd state two years later in 1819.
By 1818 the population of Alabama was well over the 60,000 target that was more customary than a fixed number under the Northwest Ordinance, about a third of that slave. The economy was booming as settlers came in growing waves. In the previous decade intensive and brutal Indian wars devastated tribal capability to resist domination of their homelands by Americans as expansionist settlement gained irreversible momentum.
The temporary territorial government established under the Northwest Ordinance incorporation model included a Governor appointed by Washington and a local legislature, which appointed a Delegate to Congress. That transitional phase was short lived, replaced in 1819 by a statehood constitution ratified by a convention comprising representatives of the territory’s 22 counties. Instead of submitting the constitution for ratification by the voters, the new constitution was sent to Congress in support of a statehood enabling act Alabama’s leaders had submitted to Congress, approved in March of 1819.
Wasting no time on a political status referendum, elections were held under the new constitution so the new statehood government could be formed.
The new Alabama legislature convened in October of 1819, and in December the President of the United States signed the admission act under which Congress admitted Alabama to the union.