… the Mexican War.
"In 1836, a piece of territory in Northern Mexico known as Texas rebelled and created its own "Lone Star Republic," the Republic of Texas. The region, though part of Mexico, had been increasingly occupied by white American settlers from the South before 1836. In addition, the region was a safe haven for escaping African-American slaves, because slavery was prohibited in Mexico. When Texas created its own republic, about 5,000 African-Americans lived in the territory. Texas, however, decided to allow slavery, and so these African-Americans lost all rights. Meanwhile, many in the United States, particularly in the South, were interested in admitting Texas as a state in the Union, which many Texans desired. This was initially rejected on the grounds that it would offset the balance of slave and free states, and that Mexico would retaliate with war. By 1845, however, Congress allowed Texas to the join the Union, which ignited conflict with Mexico.
Both the annexation of Texas and war with Mexico, however, were controversial, largely because of the issue of slavery. A dispute over Texas' southern border, however, prompted war, despite a minority of objections from Congress. Northern representatives opposed the war, believing it was a ploy to expand slave states in the Union and expand the institution westward. Famous opponents include Henry David Thoreau, who refused to pay his taxes in opposition and was imprisoned. A young Congressman from Illinois named Abraham Lincoln also voted against the war. In general, the war was waged largely by Southerners, who contributed the majority of the troops for the operation…"
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