To protect these streams, the Confederates had constructed twin forts in Tennessee just south of the Kentucky border. Fort Henry guarded the Tennessee; Fort Donelson stood menacingly on the banks of the Cumberland. On February 6 a Federal river fleet cooperating with Grant battered Fort Henry into submission. Ten days later Grant had surrounded Fort Donelson and its 12,000 defenders. Answering the Confederate commander’s request for surrender terms, Grant replied: “No terms but unconditional surrender.” Thereafter, U. S. Grant was “Unconditional Surrender” Grant. Grant’s victories brought great rejoicing in the North. Some writers consider the Henry-Donelson campaign as “the critical operation” of the Civil War. Capture of these forts assured Union control of Kentucky and Tennessee and opened Mississippi and Alabama to Federal invasion. The loss of the forts was a severe blow to Southern morale. With these successes the North had also demonstrated its ability and willingness to fight. Meanwhile, an important battle occurred farther to the west for control of Arkansas and Missouri. On March 7-8, at Pea Ridge (or Elkhorn Tavern), Ark., a Confederate army of 16,000 men attacked 12,000 Federals under Gen. Samuel R. Curtis. Most of the Confederates lacked uniforms and were armed with shotguns and squirrel rifles. This force also included 3,500 Indians of the Creek, Choctaw, Cherokee, Chickasaw and Seminole tribes. After two days of fighting, a Federal counterattack broke the Confederate “army.” With the defeat at Pea Ridge the Confederates permanently lost Missouri and northern Arkansas. [Illustration: Gunfire and fighting at Shiloh was so intense that one area of the battlefield became known as the “Hornet’s Nest.” This drawing depicts the stubborn resistance of two Federal divisions in that area.] By the end of March Grant’s army was near the Mississippi border. Just across the line Gens. Albert Sidney Johnston and Beauregard had collected a force of 40,000 Confederates. Even though most of his troops were ill-equipped, Johnston attacked Grant’s encamped forces in an effort to destroy the Federal invaders. The battle of Shiloh (April 6-7), one of the war’s most costly engagements, followed. The initial Confederate attack caught Grant by surprise, bent his line, but never broke it. Several events then swung the battle to the North’s favor. Gen. Johnston bled to death from a leg wound. Exhaustion and disorganization blunted the drive of the Southerners, and Federal artillery posted in great strength near the Tennessee River proved an effective barrier to the Confederate advance. Heavy Federal reinforcements under Gen. Don C. Buell arrived during the night. The next morning Grant counterattacked. The Confederates retreated grudgingly to Corinth, thus ending the battle. Grant’s hard-won victory cost him 13,047 casualties. The Confederates lost 10,694 soldiers, roughly one-fourth of Johnston’s forces.