After Frémont's forces halted their advance into the Valley following McDowell, Jackson next turned to defeating Banks. On May 21, Jackson marched his command east from New Market and proceeded northward. Their speed of forced marching was typical of the campaign and earned his infantrymen the nickname of "Jackson's foot cavalry". He sent his horse cavalry directly north to make Banks think that he was going to attack Strasburg, but his plan was to defeat the small outpost at Front Royal and quickly attack Banks's line of communication at Harpers Ferry. On May 23, at the Battle of Front Royal, Jackson's army surprised and overran the pickets of the 1,000-man Union garrison, capturing nearly 700 of the garrison while suffering fewer than forty casualties himself. Jackson's victory forced Banks from Strasburg into a rapid retreat towards Winchester. Although Jackson attempted to pursue, his troops were exhausted and looted Union supply trains, slowing them down immensely. On May 25, at the First Battle of Winchester, Banks's army was attacked by converging Confederate columns and was soundly defeated, losing over 1,300 casualties and much of his supplies (including 9,000 small arms, a half million rounds of ammunition, and several tons of supplies); they withdrew north across the Potomac River. Jackson attempted pursuit but was unsuccessful, due to looting by Ashby's cavalry and the exhaustion of his infantry; after a few days of rest, he followed Banks's forces as far as Harpers Ferry, where he skirmished with the Union garrison.