In March, Ulysses S. Grant was promoted to lieutenant general and appointed general-in-chief of the Union Army. He decided on a strategy of simultaneous offenses in the Eastern, Western, and Trans-Mississippi Theaters in order to grind down and ultimately defeat the Confederate armies. Grant himself planned to travel with the Army of the Potomac, commanded by George G. Meade, and the IX Corps, commanded by Ambrose Burnside, both to coordinate the two forces and to avoid the politics of Washington, D.C. (Burnside was senior to Meade and therefore would have the right to command the Army of the Potomac, so in order to retain Meade Grant had both commanders report to him. Grant concluded on May 25 that this command structure was unwieldy and placed Burnside under Meade's command.) In the Eastern Theater, Grant's forces fought General Robert E. Lee's Army of Northern Virginia in a series of battles that became known as the Overland Campaign from May to June. Although he lost heavily during the campaign and failed to gain a tactical advantage in any of the battles, Grant inflicted a larger percentage of casualties on Lee than the Union army incurred and also forced Lee into retreating closer to Richmond. Following the Battle of Cold Harbor, Grant crossed the James River and attacked the city of Petersburg, which was a vital rail center for Lee's supply lines to North Carolina. Several Union attacks on the Confederate defenses near the city in early June resulted in failure; as a result, Grant settled into a nine-month siege at Petersburg, during which he continued to move westward in order to cut Lee's supply lines.