In March 1862, the Army of the Potomac began its first large-scale offensive in the Eastern Theater of the American Civil War. McClellan landed his army at Fort Monroe. The Army then moved northwest, up the Virginia Peninsula. The Confederate defensive position on the Warwick Line caught McClellan by surprise. McClellan ordered his army to prepare for a siege of Yorktown, Virginia. Before the Union army was ready, the Confederates began a withdrawal toward Richmond. The first heavy fighting of the campaign happened in the Battle of Williamsburg. The Union troops managed some small victories, but the Confederates continued their withdrawal. An amphibious flanking movement cut off the Confederate retreat. In the Battle of Drewry's Bluff, an attempt by the U.S. Navy to reach Richmond by way of the James River failed.