Manifest Destiny was the belief that the United States had a mission to expand and that not only was it good but it was destined. Because everyone believed in manifest destiny, they wanted to push westward, no matter what. Manifest destiny also be-came known as not only expanding the territory, but also the institution of slavery. President John Quincy Adams believed so much in manifest destiny that he orchestrated the Treaty of 1818, provided for the joint occupation of the Oregon Country. He negotiated the Transcontinental Treaty in 1819, purchasing Florida from Spain and extending the U.S. border with Spanish Mexico all the way to the Pacific Ocean. And he formulated the Monroe Doctrine, which warned Europe that the Western Hemisphere was no longer open for European colonization. The introduction of so much land into the Union created nervousness between both the North and South for each had its opinions concerning land and slavery. National unity was achieved, though, because both the North and the South wanted to expand and wanted to bring more land into the country.
The introduction of Missouri to the Union was also a cause for national disunity. It was the only slave state that was created that was above the 36º30’ line. The free states grew angry because even though they got Maine to balance out, it still made them worry that the slave states would start to encroach on the free part of the Union. The Mexican War was a major cause for national disunity. The Mexican War started when President Polk sent troop into the “Disputed zone” between the newly annexed territory of Texas and Mexico. Polk wanted to provoke the Mexicans into attacking the troops so he had a reason to invade. The annexation of Texas created disunity because it added another slave state to the Union and didn’t balance out “free” and “slavery”. The Compromise of 1850 was also a cause for both unity and dis-unity. The Compromise had five laws that would balance the interests of both the Slave states and the Free states.
California was admitted as a free state, Texas received financial compensation for relinquishing claim to lands east of the Rio Grande, the territory of New Mexico was organized without any specific prohibition of slavery, the slave trade was abolished in Washington D.C., and the Fugitive Slave Law was passed, requiring all U.S. citizens to assist in the return of runaway slaves. This created a sense of dis-unity in that, even though it attempted to balance out the wishes of the slaveholders and the abolitionists it infuriated abolitionists and aided in the creation of the “Underground Railroad”. Abolitionists were willing to break the “law” to help slaves escape. Territorial expansion created more disunity than unity, which inevitably led to the start of the Civil War. Though attempts were made to appease both groups, in the end the Civil War was inevitable.