On this day in 1861, Robert E. Lee resigned his position with the United States Army in order to defend his home state of Virginia in the Confederate Army.

From Wikipedia:

Lee privately ridiculed the Confederacy in letters in early 1861, denouncing secession as "revolution" and a betrayal of the efforts of the founders. Writing to his son William Fitzhugh, Lee stated, "I can anticipate no greater calamity for the country than a dissolution of the Union." While he was not opposed in principle to secession Lee wanted all peaceful ways of resolving the differences between North and South—such as the Crittenden Compromise—to be tried first, and was one of the few to foresee a long and difficult war.

The commanding general of the Union Army, Winfield Scott, told Lincoln he wanted Lee for a top command. Lee accepted a promotion to colonel on March 28. He had earlier been asked by one of his lieutenants if he intended to fight for the Confederacy or the Union, to which Lee replied, "I shall never bear arms against the Union, but it may be necessary for me to carry a musket in the defense of my native state, Virginia, in which case I shall not prove recreant to my duty." Meanwhile, Lee ignored an offer of command from the CSA. After Lincoln's call for troops to put down the rebellion, it was obvious that Virginia would quickly secede. Lee turned down an April 18 offer by presidential aide Francis P. Blair to command the defense of Washington D.C. as a major general as he feared that the job might require him to invade the South. When Lee asked Scott if he could stay home and not participate in the war, the general replied "I have no place in my army for equivocal men."

Lee resigned from the Army on April 20, and took up command of the Virginia state forces on April 23. While historians have usually called his decision inevitable ("the answer he was born to make", wrote one; another called it a "no-brainer") given the ties to family and state, recent research shows that the choice was a difficult one that Lee made alone, without pressure from friends or family. His daughter Mary Custis was the only one among those close to Lee who favored secession, and wife Mary Anna especially favored the Union, so his decision astounded them. While Lee's immediate family followed him to the Confederacy others, such as cousins and fellow officers Samuel Phillips and John Fitzgerald, remained loyal to the Union, as did 40% of all Virginian officers.