Weaver's first chance at action came in February 1862, when the 2nd Iowa joined Brigadier General Ulysses S. Grant's army outside the Confederate Fort Donelson in Tennessee. Weaver's company was in the thick of the fight, which he described as a "holocaust to the demon of battles", and he took a minor wound in the arm. The rebels surrendered the next day, the most important Union victory of the war to date. The 2nd Iowa next joined other units in the area at Pittsburg Landing, Tennessee, to mass for a major assault deeper into the South. Confederate forces met them there, in the Battle of Shiloh. Weaver's regiment was in the center of the Union lines, in the area later known as the "hornets' nest", and were forced to retreat amid fierce fighting. The next day, the Union forces turned the tide and forced the rebels off the field in what Weaver called a "perfect rout". The carnage at Shiloh—20,000 killed and wounded—was on a scale never before seen in American warfare, and both sides learned that the war would end neither quickly nor easily.