He served in the Black Hawk War before leaving the army in 1835 to become a Mississippi planter. He commanded the Mississippi Rifles in the Mexican-American War. Davis served two terms in the Senate (1847–51, 1857–61) and was Secretary of War in the administration of President Pierce (1853–57). He left the Senate when Mississippi seceded from the Union, and in 1861 was named provisional President of the Confederacy. A year later he was elected to a six-year term. After the defeat of the Confederacy he was imprisoned (1865–67), but treason charges were not pursued. He wrote The Rise and Fall of the Confederate Government (1881) and A Short History of the Confederate States of America (1890).