In mid-May, 1863, after six months of unsuccessful attempts, Maj. Gen. Ulysses S. Grant’s Army of the Tennessee finally converged on Vicksburg, defended by a Confederate army under Lt. Gen. John C. Pemberton. Capture of the Mississippi River town was critical to Union control of the strategic river. Vicksburg was located on a high river bluff defended with artillery, and Pemberton's men had constructed a series of fortifications in an 8-mile arc surrounding the city on the landward side. After crossing the river below Vicksburg on April 30, Grant fought a series of battles as he approached Pemberton’s defenses. With the help of Navy gunboats, Grant surrounded the city by May 18. Grant launched major assaults on May 19 and 22 and was repulsed with heavy casualties, so by May 25 his army settled in for a siege. After holding out for nearly seven weeks, with no reinforcements and low on food and supplies, Pemberton surrendered on July 4. With the loss of Pemberton’s army at Vicksburg and the Union victory at Port Hudson five days later, the Union controlled the entire Mississippi River and the Confederacy was split in half.