As you may or may not know, President Lincoln asked Robert E. Lee to command the Union Army during the American Civil War. He rejected the offer, saying that accepting would be treason against his home state of Virginia.
But what if Robert E. lee had accepted the offer? How would the war have turned out?
Lee was asked to command a Union army, not all of them. Had Lee accepted, he probably would have gotten the position McDowell held in actual history. Lee was much better on the defense than the offense and his early performance in West Virgina was so poor Lee was sidelined until Jefferson Davis had no other choice.
Faced with the same problems McDowell had and requiring the same learning curve as in actual history, Lee would have lost Bull Run and been sidelined.