What if, at the Battle of Fredericksburg, in December of 1862, the Union captured it's objectives without any issue?
The Plan under General Burnsides was to use Pontoon bridges to cross the Rappahannock River and captured the town of Fredrick Virginia and the Rail line to Richmond, the CSA's capital, which would have forced Lee into an open battle.
But due to administrative bungling in Washington, they arrived fives days late, meaning Lee had already had time to move his Army to the Rappahannock's shores and set up a battle plan, this is, in my mind, what lead to the Union's defeat.
But what if it hadn't? What if, Lincoln, getting wind of such a error ordered them sent forwards? Or perhaps the Army put aside it's misgiving and mistakes and the plan did work, the Union crosses the River and takes the crossing with Richmond less then 100 miles south by rail? What happens now?