December 1845

Texas is annexed as a slave state on December 29, 1845, immediately adding hundreds of thousands of square miles to slavery's domain. The annexation was driven by Southern planters seeking fresh cotton lands and political power. It brings the US into conflict with Mexico, fueling the Mexican-American War and the explosive question of whether slavery will expand into additional conquered territory.

Congress admitted Texas as a slave state on December 29, 1845 — the largest slave state by area ever added to the Union. Texas had declared independence from Mexico in 1836 partly to preserve slavery, which Mexico had abolished. The annexation added hundreds of thousands of square miles where slavery could legally expand and shifted the balance of power in the Senate toward slave states. Texas's constitution explicitly protected slavery and prohibited the legislature from emancipating enslaved people without slaveholder consent and full compensation. Approximately 30,000 enslaved people lived in Texas at annexation; by 1860 the number exceeded 180,000.