In June 1864, Confederate Gen. Robert E. Lee sent Gen. Jubal A. Early's corps from the Richmond battlefields to the Shenandoah Valley to counter Union Gen. David Hunter's army. After driving Hunter into West Virginia, Early invaded Maryland to attack Washington, D.C., draw Union troops from Richmond, and release Confederate prisoners held at Point Lookout. On July 9, Early ordered Gen. Bradley T. Johnson's cavalry brigade eastward to free the prisoners, and the next day Johnson sent Maj. Harry Gilmer's regiment to raid the Baltimore area. Union Gen. Lew Wallace delayed Early at the Battle of Monocacy on July 9, and Federal reinforcements soon strengthened the capital's defenses. Early attacked near Fort Stevens on July 11-12 and then withdrew to the Shenandoah Valley with the Federals in pursuit, stopping them at Cool Springs on July 17-18. On the afternoon of July 17, Union forces began arriving south of here in pursuit of Early's army as it withdrew after the invasion of Maryland and attack on Washington's defenses, while Early's men were deployed across the river. The next day, after two Federal assaults upstream failed, Union Gen. George Crook ordered Col. Joseph Thoburn and his 3,250-man division to cross at Island (Parker's) Ford and march south to outflank Confederate forces at Castleman's (Snicker's) Ferry. The men began crossing at 3 p.m. and met light resistance, but captured Confederate pickets told them that almost all of Early's army was arrayed against them. Thoburn's division continued to ford there and across Parker Island to form parallel battle lines behind a stone wall along the river and another in a wheat field 100 yards inland, while Federal artillery was deployed on the heights behind and farther to the north. Across the river, Confederate Gen. John C. Breckinridge ordered Gens. Gabriel C. Wharton and Robert E. Rodes to challenge the Federals. Wharton's men pushed them back toward the river along the length of the island, and at 6 p.m. Rodes began a flanking movement from the south end of Parker Island. Joseph Thoburn, born in Northern Ireland, moved with his family to Canada and then Ohio, trained as a physician, settled in Wheeling in present-day West Virginia, and established a successful medical practice before enlisting as a surgeon in the Union army and rising to command; he was killed in action at the Battle of Cedar Creek on October 19, 1864. Despite failing to take Washington or free prisoners, Early succeeded in diverting Federal resources.