From 1855–1857, Sibley was part of the forces trying to control conflict in Bleeding Kansas, where hundreds of new settlers arrived to vote on the question of slavery, provoked by the 1854 Kansas–Nebraska Act. He took part in the Utah War, 1857–1860, and was in active service in New Mexico 1860–1861. After the outbreak of the Civil War, Sibley resigned on May 13, 1861, the day of his promotion to major in the 1st Dragoons. Native to Louisiana, he had southern sympathies and joined the Confederate States Army (CSA).