The correct answer is 2) as part of the Union Army.
Robert E. Lee began his military career as part of the Union Army.
When we think of the Civil War, one of the first characters that come to our minds is the famous leader of the Confederate Army, General Robert E.Lee. But probably you do not know that Robert E. Lee initiated its military career in West Point, New York, as a military officer. He participated in the invasion of Veracruz, Mexico. He was a renown military officer in the Army of the United States before resigned and becoming commander in chief of the Army of the State of Virginia,