from the west, preparing to bivouac, when three brigades of Gregg's division entered from the east at about 4 p.m. on June 17, surprising both sides. The resulting Battle of Aldie was a fierce mounted fight of four hours with about 250 total casualties. Munford withdrew toward Middleburg. While the fighting occurred at Aldie, the Union cavalry brigade of Col. Alfred N. Duffié arrived south of Middleburg in the late afternoon and drove in the Confederate pickets. Stuart was in the town at the time and managed to escape before his brigades under Munford and Beverly Robertson routed Duffié in an early morning assault on June 18. The primary action of the Battle of Middleburg occurred on the morning of June 19 when Col. J. Irvin Gregg's brigade advanced west from Aldie and attacked Stuart's line on a ridge west of Middleburg. Stuart repulsed Gregg's charge, counterattacked, then fell back to defensive positions to the west. On June 21, Pleasonton again attempted to break Stuart's screen by advancing on Upperville, to the west of Middleburg. The cavalry brigades of Irvin Gregg and Judson Kilpatrick were accompanied by infantry from Col. Strong Vincent's brigade on the Ashby's Gap Turnpike. Buford's cavalry division moved northwest against Stuart's left flank, but made little progress against Grumble Jones's and John R. Chambliss's brigades. The Battle of Upperville ended as Stuart conducted a fierce fighting withdrawal and took up a strong defensive position in Ashby's Gap. After successfully defending his screen for almost a week, Stuart found himself motivated to begin the most controversial adventure of his career, Stuart's raid around the eastern flank of the Union Army. Hooker's significant pursuit with the bulk of his army began on June 25, after he learned that the Army of Northern Virginia had crossed the Potomac River. He ordered the Army of the Potomac to cross into Maryland and concentrate at Middletown (Slocum's XII Corps) and Frederick (the rest of the army, led by Reynolds's advance wing—the I, III, and XI Corps). The invasion of Pennsylvania President Lincoln issued a proclamation calling for 100,000 volunteers from four states to serve a term of six months "to repel the threatened and imminent invasion of Pennsylvania." Pennsylvania Governor Andrew Curtin called for 50,000 volunteers to take arms as volunteer militia; only 7,000 initially responded, and Curtin asked for help from the New York State Militia. Gov. Joel Parker of