Secretary of War Simon Cameron had been under fire for some time over hints of corruption in his department. He had also placed himself at odds with President Lincoln and others when the first drafts of his December 1861 report suggested the raising of black military units. In the same letter accepting his resignation, Lincoln appointed him as ambassador to Russia. Lincoln then appointed Edwin McMasters Stanton as Cameron's successor. (By John Osborne)
Erwin Bradley, Simon Cameron, Lincoln’s Secretary of War: A Political Biography (Philadelphia: University of Pennsylvania Press, 1966), 206.