In June 1863, the Army of the Cumberland set out for Tullahoma, with the strategic purpose of holding Bragg's attention and preventing him from sending forces to the relief of Vicksburg. XIV Corps, now under the command of General George Henry Thomas, was ordered to probe Hoover's Gap, one of several passes through the Highland Rim, which the Union would need to secure to make its advance. The brigade of Colonel John T. Wilder was sent in advance as mounted infantry and drove the Confederates back, seizing the high ground of the gap and refusing orders to fall back. After Wilder's brigade successfully repulsed several Confederate counterattacks, the 21st Wisconsin was one of several units sent up to reinforce Wilder on the evening of June 24. With the Union in firm possession of Hoover's Gap, the Confederates were forced to evacuate Middle Tennessee and retrench at Chattanooga, forming a new defensive line along the Tennessee River.