With access to the Gulf of Mexico secured, national attention was immediately drawn back to Louisiana beyond the Mississippi, for the next big national issue had been waiting in the wings. Louisiana was to become the stage on which the first act in the drama that would culminate in the Civil War, would be played out. During the next sixty years the principles were to be defined there, the players aligned, and decisions tested. The prologue had been the Northwest Ordinance of 1787, which, among other things, forever outlawed slavery from the Northwest Territory.
The next scene was set in Missouri, which sought statehood in 1820. The political problem was, how to maintain a balance of Northern and Southern voting power in the Senate. The solution—the "Compromise"—was to admit Missouri as a slave state, but thenceforth to prohibit slavery north of 36 degrees, 30 minutes north, in the lands acquired through the Louisiana Purchase.
Meanwhile, the question of the actual boundaries of Louisiana continued to demand answers. Lewis and Clark weren't much help. They crossed the Continental Divide, which separates the Missouri River drainage from the Columbia's, a total of six times, but never even tried to find the latitude or longitude at their crossing points. They made celestial observations at Camp Fortunate, which was about 30 miles east of Lemhi Pass, but their calculations placed them some 25 or 30 miles south of their actual location. Clark's map of the Northwest, and of the Rockies where they crossed them, was nonetheless remarkably accurate, considering the time and the place, but it was essentially anecdotal by 20th-century standards. It would remain for railroad surveyors after 1850 to produce more precise records of a few points on the western boundary of Louisiana.
In August of 1805 General Wilkinson, commander of U.S. forces in the Mississippi Valley, had sent Lieutenant Zebulon Pike to find the sources of the Mississippi River. A year later he sent Pike to explore the Southwest as far as the Rio Grande Valley.
Before the Adams-Onis treaty of 1821, some people held that the Rio Grande River certainly was the southwestern boundary, while Spanish authorities contended that the real western boundary of Louisiana was the ninety-third degree of longitude! On the other hand, a French map of 1762 showed Louisiana extending all the way to the Pacific Ocean, encompassing the area later to become Oregon Territory, and in 1821 Francois Barbe-Marbois published a map of the United States showing a similar boundary from the Rocky Mountains to the Pacific Ocean approximately along the 42nd parallel. The British, of course, disagreed, and ownership of Oregon Territory remained a point of sometimes violent contention between them and the Americans, especially among the fur traders who did business there.1
After the American conquest of northern Mexico in 1848, Eastern backers of a southern transcontinental railroad found the best route lay through some land that still belonged to Mexico. James Gadsden, the U.S. minister to Mexico, engineered the deal, acquiring 30,000 square miles of prime agricultural valley for $10 million—a little under 53 cents an acre.2
2. Louisiana Territory, without "the Floridas," had cost the U.S. about three cents per acre in 1803. In 1999, a single, 85,000-acre parcel of ranch land in central Montana sold to a private party for $23,000,000—about $270 per acre.