On Monday, 17 December, 1860, both Mr. Thompson & Judge Black informed me that they had held conversations with General Cass on the subject of his resignation & that he had expressed a desire to withdraw it & return to the Cabinet. I gave this no encouragement. His purpose to resign had been known for several days, & his actual resignation had been prepared three days before it was delivered to me. The world knew all about it, & had he returned, the explanation would have been very embarrassing. Besides, I knew full well that his fears would have worried the administration as well as himself in the difficult times which were then upon us. His great error was that he would assume no responsibility which he could possibly avoid.
SOURCE: John Bassett Moore, Editor, The Works of James Buchanan, Comprising his Speeches, State Papers, and Private Correspondence, Volume 11: 1860-1868, p. 67