"It is good that war is so terrible; otherwise we should grow too fond of it." (Robert E. Lee)
The Army of the Potomac had pushed Robert E. Lee's army out of Maryland in September 1862 after the Battle of Antietam, but President Lincoln and his War Department wanted the army to continue going after the Army of Northern Virginia after it retreated back into Virginia. When George B. McClellan refused to do it, Lincoln fired him and installed Ambrose E. Burnside as the new commander. Burnside, who didn't believe himself capable of commanding the Army of the Potomac, only took the job because he was told Fighting Joe Hooker would get the spot if he refused.
With Washington urging Burnside to advance against Lee, Burnside launched an ill-fated operation across the Rappahannock River near Fredericksburg in December 1862. From December 12-13, Burnside struggled to get his army across the river while it was under fire from Confederates in Fredericksburg, and things only got worse when they did.
As injured Northern soldiers lay freezing and dying on the field that night, the Northern Lights made a rare appearance. Southern soldiers interpreted it as a favorable omen from God and mentioned them frequently in their diaries, while Northern soldiers who saw something far less divine sparsely mentioned them. The following morning, Burnside extricated his army back behind the river, ending the fighting in 1862.