People migrated for different reasons, but primarily for economic motives. People moved into Mexican territory because the Mexican government was offering cheap land, though the issue of slavery (which was outlawed in Mexico when they gained independence from Spain) quickly became an issue, as settlers wanted to raise cotton there. This, in fact, was one of the major issues that led to the Texas Revolution. As for California and Oregon, the bulk of migration took place immediately before and after the Mexican War, with the peak being the years of the gold rush in California. Some people went to Oregon for very different reasons, as the example of Marcus and Narcissa Whitman demonstrates. They went to proselytize to Indians in the Northwest. But by far, most people went west for increased economic opportunity.