This is the region bordered today by Mexico on the south, the Pacific Ocean on the west, the continental divide on the east, and the 42nd parallel in the north. The continental divide was the original border between the Louisiana Purchase and New Spain back in 1803. The northern and southern ends of this border, however, were not firmly established until the Adams-Onis Treaty of 1819. This set up a zig-zag border following rivers and lines of longitude in the south and limiting Spanish claims in the north to those lands below the 42nd degree of latitude. This large area of land was in 1840 part of Mexico, but in less than ten years it would be United States territory.
Mexico had achieved independence from Spain in 1821. At that time the northern part of the country was quite sparsely populated. There were a series of settlements in the Mexican province of New Mexico, up the Rio Grande and including Albuquerque and Santa Fe. There were also a number of settlements built up around a chain of missions running along the California coast from near today’s San Diego to just north of the San Francisco Bay area.
There was even less settlement in the Mexican province of Texas. The Mexican government did not have the resources nor inclination to wield much influence there, especially in keeping the Comanches under control, so the local settlers were given considerable autonomy. The Mexicans began to allow Americans to settle in Texas, hoping they would help provide a buffer between Mexico and the expansionist United States. Beginning in the 1820s, the Mexican government set up twenty-four “empresarios” of new settlers in Texas, most from the United States and including Stephen F. Austin’s settlement along the Brazos River.
As should have been obvious, this was letting the wolf in to guard the hen house, and by the 1830s, tensions began to escalate significantly between the Mexican government and the Americans. The political situation in Mexico was turbulent throughout this period and in 1835, General Santa Anna overthrew the Mexican constitution and set up a dictatorship. This was all the Texans needed to go into open revolt, declaring and then winning their independence the following year, forming the Republic of Texas (1836-45).
Texas was able to govern itself in this period, even though Mexico never recognized its independence. Still, there was considerable pressure for Texas to become part of the United States. This was because of the strong ties between the Anglo-Texans and the United States, and because of the desire of Southerners to add more slave states to the country (which at the time were limited by the 1820 Missouri Compromise to lands below 36°30’ parallel). At the end of 1845, Texas was officially annexed into the United States as the 28th state.
The border between Texas and Mexico had never been agreed to, with the Texans claiming as far as the Rio Grande and the Mexicans claiming lands about 150 miles further north, up to the Nueces River. The United States took up the Texan claim and President Polk decided to push the issue by sending troops into the disputed land between the two rivers. Not surprisingly, Mexico attacked these troops and war was declared.
In less than a year and a half, the Americans had captured Mexico City, as well as many other Mexican cities and much territory, forcing the Mexicans to sue for peace. The war was ended with the February 2, 1848 Treaty of Guadalupe Hidalgo. In exchange for just over $18 million, the United States received from Mexico acknowledgement of its undisputed control of Texas, and all of what had been Mexican Upper California and New Mexico.
With this treaty, the United States increased its size by about 20% over what it had been in 1840. At the end of this decade, by the settlements of the Oregon question and the Mexican-American War, the United States achieved essentially the overall shape of today’s lower 48 states, all except for slightly over 29,000 square-miles in what is now southern Arizona and New Mexico. That final piece of the puzzle wasn’t added until the second half of the century, but the country had grown to reach close to its final continental shape in the first half-century, with a doubling in size in 1803 and the additional of about the same amount again just before mid-century.
Click here to read about the final changes to the trans-Mississippi West during this decade.