Lincoln established the basic facts of the Missouri Compromise and went on a tour of early regulation of slavery by the Congress, all the way back to the Confederation. But Lincoln had spoken for only two sections of the country: the Northwest where he stood and the Louisiana Purchase. The country had more land than that to exclude or include slavery on. In discussing that he reveals an interesting wrinkle in the nation’s territorial expansion:
Texas principally south of the line, and West of Arkansas; though originally within the purchase from France, had, in 1819, been traded off to Spain, in our treaty for the acquisition of Florida. It had thus become a part of Mexico. Mexico revolutionized and became independent of Spain. American citizens began settling rapidly, with their slaves in the southern part of Texas.
I had to read that a few times and do some checking before I fully understood it. Texas a part of the Louisiana Purchase? Not in my history textbook!
But at the time, the US claimed that it had bought at least large sections of Texas. Nobody making the claims knew where the Louisiana Purchase really ended. It began at the Mississippi River, but went off into blank sections of the map inhabited only by little-known Indians and, somewhere far off west, Spaniards. The French had not settled it anywhere in great numbers, except around New Orleans and St. Louis. Napoleon properly sold Thomas Jefferson the claim to the western half of the Mississippi watershed more than the land itself. That could, theoretically, include everything between the continental divide and the Gulf of Mexico. Spain had a competing claim in the Viceroyalty of New Spain, which went down from the Oregon Country all the way to the Viceroyalty of New Grenada at roughly the northern border of Panama.
The parting Lincoln refers to happened in the Adams-Onís Treaty. It settled several longstanding territorial disputes with Spain, which insisted until then that its possession of Florida ran all the way to the Mississippi and that when Jefferson bought Louisiana, he bought only the city itself and a narrow band of territory around the river. The initial American claim put Louisiana’s western border at the Rio Grande, halfway through modern New Mexico, but the United States eventually opted for the Sabine River, the modern line between Louisiana and Texas, but Spain insisted on the Arroyo Hondo, now the Calcasieu River. The disputed territory, which both sides informally agreed to treat as neutral, drew settlers, squatters, and various criminal interests that caused some problems for both countries.
Over in Florida, the United States had a similar problem. Spanish authority did not reach very far on the ground. This made Florida a haven for Indians that liked to raid across the border and runaway slaves who could happily vanish into the wilderness. Andrew Jackson took that as an excuse to move an army across the border to fight the Seminoles and seized some Spanish forts along the way. Washington refused to disavow his actions or recall him, which put Spain in quite a bind at a time when it needed money, faced rebellions in its Latin American possessions, and had just come out of the very damaging Napoleonic Wars. Better to come to the table and cut its losses in exchange for some cash. So the sticky-fingered Americans got Florida and the Sabine River boundary. Spain threw in its claims to the Oregon Country too. In exchange, the United States surrendered its nominal claims to Texas and agreed on a boundary that ceded some little-known land between the Arkansas and Red rivers west of 100° longitude to Spain.
Of course, Texas came back into the Union amid much controversy later on:
Soon they revolutionized against Mexico, and established an independent government of their own, adopting a constitution, with slavery, strongly resembling the constitutions of our slave states. By still another rapid move, Texas, claiming a boundary much further West, than when we parted with her in 1819, was brought back to the United States, and admitted into the Union as a slave state. There then was little or no settlement in the northern part of Texas, a considerable portion of which lay north of the Missouri line; and in the resolutions admitting her into the Union, the Missouri restriction was expressly extended westward across her territory. This was in 1845, only nine years ago.
See, Douglas? Even at that late date, we drew the Missouri Compromise line all over again. We drew it that way even if it meant cutting off a bit of a state, though with the proviso that the line would only enter legal force in the event that parts of Texas got divided off into new states.