Trick question. He was with the Confederate Army from Apr 12, 1861 – May 9, 1865.
Robert Edward Lee (January 19, 1807 – October 12, 1870) was an American general known for commanding the Confederate Army of Northern Virginia in the American Civil War from 1862 until his surrender in 1865. The son of Revolutionary War officer Henry "Light Horse Harry" Lee III, Lee was a top graduate of the United States Military Academy and an exceptional officer and military engineer in the United States Army for 32 years. During this time, he served throughout the United States, distinguished himself during the Mexican–American War, and served as Superintendent of the United States Military Academy.