Following his withdrawal from Rocky Face Ridge, the first battle in Maj. Gen. William T. Sherman's campaign against Atlanta, Confederate Gen. Joseph E. Johnston established a four-mile long defensive position protecting the Western & Atlantic Railroad northwest of nearby Resaca, where the railroad crossed the Oostanaula River. On May 13, 1864, Sherman tested the Rebel lines with little result. The next day, a full-scale battle erupted. Sherman attacked the Confederate center with the Army of the Ohio and the Army of the Cumberland; Johnston replied late that afternoon with a counter-attack against the Union left. Neither attack was decisive. On the 15th, Sherman renewed his assault on Johnston’s right. Meanwhile, a division of Federal troops crossed the Oostanaula at Lay’s Ferry south of the Confederate line, effectively flanking Johnston out of his entrenchments and forcing him to withdraw. Sherman's overpowering advantage in manpower and his flanking technique would continue be used against Johnston repeatedly during the Atlanta campaign, usually with success.