Why was Andrew Johnson important in the U.S. Civil War?

Andrew Johnson was the military governor of Tennessee after the state fell into Union hands in early 1863.  He was a moderate Democrat from East Tennessee who pulled a gun on some secessionists who tried to stop his train to Nashville to take the oath of office.  While in office, he made sure that Nashville was a major Union supply depot for Grant's army at Vicksburg and during the early days of Sherman's Atlanta campaign.  Johnson's main contribution to the Civil War was how he helped to balance Lincoln's ticket in 1864, since if the Peace Democrats under McClellan won, the Confederates might have gained a negotiated peace and independence.  Johnson was the first vice-president to take over after his predecessor was assassinated and he failed to contend with the Radical Republicans in Congress, who did not trust his Democratic leanings or his Southern roots.