As provisional governor of Missouri during the Civil War, Hamilton Gamble (1798--1864) worked closely with the Lincoln administration to keep the state from seceding from the Union. Without Gamble and other loyal Unionist governors, the war in the West might have been lost. Dennis Boman's full-scale account of Gamble's life tells the little-known story of a prominent frontier lawyer who became chief justice of the Missouri Supreme Court and boldly dissented in the infamous Dred Scott decision. Revealing how Gamble, one of the wealthiest and most renowned citizens of pre--Civil War Missouri, fought to end slavery and to protect the integrity of the Union, Lincoln's Resolute Unionist corrects prevailing notions about solidarity among the South's antebellum elite on these issues. The slaveholding border state of Missouri figured greatly in the sectional crisis from the time of its controversial admission to the Union up through the war itself, when it was the site of internecine battles between Unionists and Confederates. The complexities of the period and of the political alliances formed then emerge clearly in Boman's biography of Gamble. A fundamental conservatism -- Gamble believed judges should interpret, not make, law -- led the southern slave owner to dissent from his colleagues' proslavery decision in Scott v. Emerson. These same principles, along with Gamble's Whig affiliation and Christian convictions, made firm his antisecessionist stance despite his proslavery predilections. Boman provides a groundbreaking analysis of Lincoln's involvement in Missouri's affairs, including his assistance to Gamble in maintaining security and passing a state ordinance for gradual emancipation. Lincoln's Resolute Unionist brings to light in a compelling fashion the meaning -- and the drama -- of the life of a key figure at a critical time in American history.