 However, despite anticipating that Grant might shift across the James, Lee was taken by surprise when it occurred.On June 12 the Army of the Potomac finally disengaged to march southeast to cross the James and threaten Petersburg, a crucial rail junction south of Richmond.The Battle of Cold Harbor was the final victory won by Lee's army during the war (part of his forces won the Battle of the Crater the following month, during the Siege of Petersburg, but this did not represent a general engagement between the armies), and its most decisive in terms of casualties.