The debate about slavery in other regions including the West started the Missouri Comprise in 1820 which helped solve the problems by maintaining a balance between free and slave states. Maine was named a free state making the number of free and slave states equal.
The compromis established a boundary line where slavery would be prohibited in the Louisiana Territory north of the line.
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So, in 1854, Senator Stephen A. Douglas suggested creating Kansas and Nebraska, but the issue of slavery remained. Douglas proposed "popular sovereignty," letting settlers decide on slavery. This led to conflict in Kansas, with Missourians voting fraudulently for slavery.
The turmoil in Kansas foreshadowed the national Civil War over slavery. The debate over slavery in the West ultimately became a critical factor in the nation's unity.
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After the Missouri Compromise, as Americans continued migrating westward, many settled in the Oregon Territory and Mexican territories like California, New Mexico, and Texas. Texas, after gaining independence from Mexico, joined the U.S. as a slave state, challenging the balance set by the Missouri Compromise. It wasn't until James K. Polk's presidency in 1844 that Texas, a slave state, and Oregon, a free state, joined the Union.
However, the repossession of these territories and the Mexican-American War highlighted the ongoing sectional conflict over slavery expansion, as seen through David Wilmot's unsuccessful proviso to restrict slavery in acquired Mexican territories, revealing the tensions surrounding westward expansion.