Finding he could not break Lee's line of defense at Spotsylvania, Grant turned southward and moved to the North Anna River a dozen miles closer to Richmond. An attempt was made by Grant to get Lee to fight out in the open by sending an individual II Corps on the west bank of the Mattaponi River. Rather than take the bait, Lee anticipated a second right flank movement by Grant and retreated to the North Anna River in response to the Union V and VI corps, withdrawing from Spotsylvania. During this time, many Confederate generals, including Lee, were incapacitated due to illness or injury. Lee, stricken with dysentery, was unable to take advantage of an opportunity to seize parts of the Army of the Potomac. After a series of inconclusive minor battles at North Anna on May 23 and 24, the Army of the Potomac withdrew 20 miles southeast to important crossroads at Cold Harbor. From June 1 to 3, Grant and Lee fought each other at Cold Harbor with the heaviest Union casualties on the final day. Grant's ordered assault on June 3 was disastrous and lopsided with 6,000 Union casualties to Lee's 1,500. After twelve days of fighting at Cold Harbor, total casualties were 12,000 for the Union and 2,500 for the Confederacy. On June 11, 1864, Grant's Army of the Potomac broke away completely from Robert E. Lee, and on June 12 secretly crossed the James River on a pontoon bridge and attacked the railroad junction at Petersburg. For a brief time, Robert E. Lee had no idea where the Army of the Potomac was.