Until 1845, it had seemed likely that slavery would be confined to the areas where it already existed. It had been given limits by the Missouri Compromise in 1820 and had no opportunity to overstep them. The new territories made renewed expansion of slavery a real likelihood.  

Many Northerners believed that if not allowed to spread, slavery would ultimately decline and die. To justify their opposition to adding new slave states, they pointed to the statements of Washington and Jefferson, and to the Ordinance of 1787, which forbade the extension of slavery into the Northwest. Texas, which already permitted slavery, naturally entered the Union as a slave state. But California, New Mexico and Utah did not have slavery, and when the United States prepared to take over these areas in 1846, there were conflicting suggestions on what to do with them.  

Extremists in the South urged that all the lands acquired from Mexico be thrown open to slave holders. Antislavery Northerners, on the other hand, demanded that all the new regions be closed to slavery. One group of moderates suggested that the Missouri Compromise line be extended to the Pacific with free states north of it and slave states to the south. Another group proposed that the question be left to "popular sovereignty," that is, the government should permit settlers to enter the new territory with or without slaves as they pleased and, when the time came to organize the region into states, the people themselves should determine the question.  

Southern opinion held that all the territories had the right to sanction slavery. The North asserted that no territories had the right. In 1848 nearly 300,000 men voted for the candidates of a Free Soil Party, who declared that the best policy was "to limit, localize and discourage slavery." The Midwestern and border state regions -- Maryland, Kentucky, Missouri -- were even more divided, however, with many favoring popular sovereignty as a compromise.  

In January 1848 the discovery of gold in California precipitated a headlong rush of more than 80,000 settlers for the single year 1849. California became a crucial question, for clearly Congress had to determine the status of this new region before an organized government could be established. The hopes of the nation rested with Senator Henry Clay, who twice before in times of crisis had come forward with compromise arrangements. Now once again he halted a dangerous sectional quarrel with a complicated and carefully balanced plan.  

His compromise (as subsequently modified in Congress) contained a number of key provisions: that California be admitted as a state with a free-soil (slavery-prohibited) constitution; that the remainder of the new annexation be divided into the two territories of New Mexico and Utah and organized without mention of slavery; that the claims of Texas to a portion of New Mexico be satisfied by a payment of $10 million; that more effective machinery be established for catching runaway slaves and returning them to their masters; and that the buying and selling of slaves (but not slavery) be abolished in the District of Columbia. These measures -- known in American history as the Compromise of 1850 -- were passed, and the country breathed a sigh of relief.  

For three years, the compromise seemed to settle nearly all differences. Beneath the surface, however, tension grew. The new Fugitive Slave Law deeply offended many Northerners, who refused to have any part in catching slaves. Moreover, many Northerners continued to help fugitives escape, and made the Underground Railroad more efficient and more daring than it had been before.