On June 2, 1864, Ulysses S. Grant post­poned until the following morning an assault on Confederate lines near Cold Harbor planned for that afternoon be­cause of delays in positioning troops. In the meantime, Confederate forces strengthened their lines, and the assault became a slaughter that haunted Grant for the rest of his life.   Thus began a summer of frustration for the general-in-chief of the U.S. Army. By failing to press their advantage, Major General William F. “Baldy” Smith and Major General Ambrose E. Burnside in a six-week period fumbled two genuine opportunities to defeat Lee’s army. An­noyed by the constant calls of Major Generals William S. Rosecrans and Sam­uel R. Curtis for reinforcements in Missouri and Kansas, he wrote that “I am satisfied you would hear the same call if they were stationed in Maine.”   Confederate forces commanded by Lieutenant General Jubal A. Early again threatened Washington, forcing Grant to send two army corps to defend the capital and to push the invaders back into the Shenandoah Valley. The pressure took its toll on his health: migraine headaches followed such setbacks as the battle of the Crater.