How was the United States able to gain more territory / land and become larger in the 1800s?

In 1debate over the issue, Kentucky Senator Henry Clay proposed another compromise. It had four parts: first, California would enter the Union as a free state; second, the status of slavery in the rest of the Mexican territory would be decided by the people who lived there; third, the slave trade (but not slavery) would be abolished in Washington, D. C.; and fourth, a new Fugitive Slave Act would enable Southerners to reclaim runaway slaves who had escaped to Northern states where slavery was not allowed.

Bleeding Kansas

But the larger question remained unanswered. In 1854, Illinois Senator Stephen A. Douglas proposed that two new states, Kansas and Nebraska, be established in the Louisiana Purchase west of Iowa and Missouri. According to the terms of the Missouri Compromise, both new states would prohibit slavery because both were north of the 36º30' parallel. However, since no Southern legislator would approve a plan that would give more power to "free-soil" Northerners, Douglas came up with a middle ground that he called "popular sovereignty": letting the settlers of the territories decide for themselves whether their states would be slave or free.

Northerners were outraged: Douglas, in their view, had caved to the demands of the "slaveocracy" at their expense. The battle for Kansas and Nebraska became a battle for the soul of the nation. Emigrants from Northern and Southern states tried to influence the vote. For example, thousands of Missourians flooded into Kansas in 1854 and 1855 to vote (fraudulently) in favor of slavery. "Free-soil" settlers established a rival government, and soon Kansas spiraled into civil war. Hundreds of people died in the fighting that ensued, known as "Bleeding Kansas."

A decade later, the civil war in Kansas over the expansion of slavery was followed by a national civil war over the same issue. As Thomas Jefferson had predicted, it was the question of slavery in the West-a place that seemed to be the emblem of American freedom-that proved to be "the knell of the union."