During Jackson's Valley Campaign of 1862, Garnett's military career took a downward turn at the First Battle of Kernstown in March. Jackson marched his army to intercept a portion of the Union Army under Maj. Gen. Nathaniel P. Banks. On March 23, Jackson's cavalry commander, Col. Turner Ashby, brought faulty intelligence that the retreating Union division of Brig. Gen. James Shields had four regiments in the rear outside Winchester, Virginia. Since that force was of comparable size to Jackson's, he ordered Garnett and the Stonewall Brigade to attack. Unfortunately, Shields had a full infantry division on hand, almost 9,000 men, twice the size of Jackson's force. The attack went badly and Garnett, finding his brigade low on ammunition and surrounded by forces attacking from three sides, ordered a retreat. Jackson was infuriated and accused Garnett of disobeying orders, since he did not have Jackson's permission to retreat. Jackson, well known as a strict disciplinarian, arrested Garnett for "neglect of duty" on April 1 and relieved him of command. Garnett's court-martial started in August 1862, with only Jackson and his aide giving testimony. However the trial was suspended due to the start of Gen. Robert E. Lee's Northern Virginia Campaign and the Second Battle of Bull Run that August.