His resignation as secretary of war, on December 29, 1860, was precipitated by the refusal of Buchanan to order Major Robert Anderson to abandon Fort Sumter, which eventually led to the start of the war.
Hans tilbagetræden som krigsminister den 29. december blev fremskyndet af, at Buchanon nægtede at beordre major Robert Anderson til at opgive Fort Sumter, hvilket senere ledte til krigens udbrud.