From sonofthesouth.net :
About a month later Lee was summoned to Washington to report to General Scott and reached the capital on the 1st of March, only a few days before the inauguration of Lincoln. He was then just fifty-four years of age, and dating from his cadetship at West Point had been in the military service of the government about thirty-six years. He had reached the exact prime of maturity; in form, features, and general bearing the type of magnificent manhood; educated to thoroughness; cultivated by extensive reading, wide experience, and contact with the great men of the period; with a dauntless bravery tested and improved by military perils in many battles; his skill in war recognized as of the highest order by comrades and commanders; and withal a patriot in whom there was no guile and a man without reproach. Bearing this record and character, Lee appeared at the capital of the country he loved, hoping that wisdom in its counsels would avert coercion and that this policy would lead to reunion. Above all others he was the choice of General Scott for the command of the United States army; and the aged hero seems to have earnestly urged the supreme command upon him. Francis P. Blair also invited him to a conference and said, "I come to you on the part of President Lincoln to ask whether any inducement that he can offer will prevail on you to take command of the Union army." To this alluring offer Lee at once replied courteously but candidly that though "opposed to secession and deprecating war he would take no part in the invasion of the Southern States." His resignation followed at once, and repairing to Virginia, he placed his stainless sword at the service of his imperiled State and accepted the command of her military forces. The commission was presented to him in the presence of the Virginia convention on April 23, 1861, by Mr. Janney, the president of that body, with ceremonies of great impressiveness, and General Lee entered at once upon duties which absorbed his thought and engaged his heart. The position thus assigned confined him at first to a narrowed area, but he diligently organized the military strength of Virginia and surveyed the field over which he foresaw the battles for the Confederacy would be fought. As late as April 25 he wrote, "No earthly act would give me so much pleasure as to restore peace to my country, but I fear it is now out of the power of man, and in God alone must be our trust. I think our policy should be purely on the defensive, to resist aggression and allow time to allay the passions and permit reason to resume her sway."
Thus Robert E. Lee became the only person in recorded history to be offered the command of both armies in a major war.