On June 17, Thomas fought with his brigade in the Battle of Aldie, driving Confederate cavalrymen into and, over the next several days, beyond Middleburg. They then clashed again in the Battle of Upperville, and attacked the rear of Stuart's army near Westminster (June 30) before heading for Manchester, Hanover Junction, and on into Adams County, Pennsylvania. While fighting with his regiment in day three of the Battle of Gettysburg, Pennsylvania on July 3, 1863, Thomas led a charge of Union men on the left side of 12,500 Confederate cavalrymen and infantry troops, engaging in a "hacking" battle with the enemy and helping to inspire the CSA to retreat, according to historians Edward Longacre and Thomas Holbrook. Recalling the day's events in his 1889 memoir, Thomas noted that: