[4] He led a brigade of Virginia cavalry in the Second Battle of Bull Run in August 1862, and the early part of the Maryland Campaign in September 1862.

Prior to the Battle of Antietam, having a very mixed relation with his superiors, he was relieved of his command and ordered to North Carolina to recruit and train new cavalry regiments.

[6] After training and sending north several cavalry units, during the Gettysburg Campaign Robertson was back with the Army of Northern Virginia.

He fought in Stuart's delaying actions in the Loudoun Valley at Middleburg and Upperville.

Due to seniority, for parts of the campaign Robertson directed both his and "Grumble" Jones's brigades.

Robertson helped cover Lee's retreat following the Battle of Gettysburg, the brigade having suffered heavy losses during the campaign.