Due to its central location, Tennessee was a crucial state during the war and saw more military engagements than any state except Virginia. After Abraham Lincoln was elected president in 1860, secessionists in the state government led by Governor Isham Harris sought voter approval to sever ties with the United States, which was rejected in a referendum by a 54–46% margin in February 1861. After the Confederate attack on Fort Sumter in April and Lincoln's call for troops in response, the legislature ratified an agreement to enter a military league with the Confederacy on May 7, 1861. On June 8, with Middle Tennesseans having significantly changed their position, voters approved a second referendum on secession by a 69–31% margin, becoming the last state to secede. In response, East Tennessee Unionists organized a convention in Knoxville with the goal of splitting the region to form a new state loyal to the Union. In the fall of 1861, Unionist guerrillas in East Tennessee burned bridges and attacked Confederate sympathizers, leading the Confederacy to invoke martial law in parts of the region. In March 1862, Lincoln appointed native Tennessean and War Democrat Andrew Johnson as military governor of the state. General Ulysses S. Grant and the U.S. Navy captured the Tennessee and Cumberland rivers in February 1862 at the battles of Fort Henry and Fort Donelson. Grant then proceeded south to Pittsburg Landing and held off a Confederate counterattack at Shiloh in April in what was at the time the bloodiest battle of the war. Memphis fell to the Union in June after a naval battle on the Mississippi River. Union strength in Middle Tennessee was tested in a series of Confederate offensives beginning in the summer of 1862, which culminated in General William Rosecrans's Army of the Cumberland routing General Braxton Bragg's Army of Tennessee at Stones River, another one of the war's costliest engagements. The next summer, Rosecrans's Tullahoma campaign forced Bragg's remaining troops in Middle Tennessee to retreat to Chattanooga with little fighting. During the Chattanooga campaign, Confederates attempted to besiege the Army of the Cumberland into surrendering, but reinforcements from the Army of the Tennessee under the command of Grant, William Tecumseh Sherman, and Joseph Hooker arrived. The Confederates were driven from the city at the battles of Lookout Mountain and Missionary Ridge in November 1863. Despite Unionist sentiment in East Tennessee, Confederates held the area for most of