7) was one of the ablest and most successful commanders of cavalry in any Civil War army. Pennsylvania-born, West Point-educated, and deeply experienced in cavalry operations prior to the conflict, his career personified that of the typical cavalry officer in the mid-nineteenth-century American army. Gregg achieved distinction on many battlefields, including those during the Peninsula, Chancellorsville, Gettysburg, Bristoe, Overland, and Petersburg campaigns, ultimately gaining the rank of brevet major general as leader of the Second Division, Cavalry Corps, Army of the Potomac.

The highlight of his service occurred on July 3, 1863, the climactic third day at Gettysburg, when he led his own command as well as the brigade of Brig. Gen. George Armstrong Custer in repulsing an attempt by thousands of Confederate cavalry under the legendary J. E. B. Stuart in attacking the right flank and rear of the Union Army while Pickett's charge struck its front and center.

Historians credit Gregg with helping preserve the security of his army at a critical point, making Union victory inevitable. Unlike glory-hunters such as Custer and Stuart, Gregg was a quietly competent veteran who never promoted himself or sought personal recognition for his service. Rarely has a military commander of such distinction been denied a biographer's tribute. Gregg's time is long overdue.