Early in the nineteenth century the people of the United States, and especially those of the south-west, became interested in that part of Mexico known as Texas. The Louisiana purchase was made in 1803. Settlers went immediately into that region along the Mississippi River. The expansionist movement then grew rapidly as we have seen, and was the major cause of the War of 1812. American settlers pushed into Mexico and soon got control of that section now known as Texas, where there were few Mexicans. These citizens of the United States went there on the assumption that Texas would some day become a part of the United States. Much of Texas was suited for the raising of cotton,--hence slavery was profitable. In 1827 Mexico passed a law providing for the gradual abolition of slavery. The people of Texas interested in slavery, resented this, as did the pro-slavery factions in the United States. Sentiment in Texas for secession crystalized rapidly, and in 1836 Texas seceded from Mexico, later asking to be annexed to the United States. Some of the anti-slavery groups opposed this annexation which would increase the slave territory. In her constitution of 1837 Texas legalized slavery. It was not until 1845 that Texas was admitted as a state. Polk of Tennessee, an ardent expansionist, was elected President by the Democrats in 1844. "Manifest Destiny" had then become the slogan of the Democratic party. Accordingly, President Tyler secured the annexation of Texas as a state just before his term of office closed in 1845, a few days before he was succeeded by Polk. Texas in revolt from Mexico claimed more territory than she had possessed while a Mexican state. Her southern boundary had then been the Nueces River, but after revolting, she laid claim down to the Rio Grande River. This area between the Nueces River and the Rio Grande was sparsely settled, but its inhabitants were Mexicans and included the Mexican settlements at the mouth of the Rio Grande. Polk did not desire war but he was eager to acquire this disputed territory. He sent John Slidell, of Louisiana, as minister to Mexico to induce Mexico to accept the Rio Grande as the southern boundary of Texas rather than the Nueces River, which had been the southern boundary of Texas while a Mexican province. Slidell was also instructed to buy from Mexico the territory now comprising the states of New Mexico, California, Arizona, Utah, Nevada, and part of Colorado, all of which was then a part of Mexico. Mexico, however, refused to receive Slidell or consider disposing of that territory, on the ground that the United States had annexed Texas although Mexico had not acknowledged its independence.