Joel H. Silbey, "Cass, Lewis," American National Biography Online, February 2000, http://www.anb.org/articles/04/04-00213.html.
In 1857 President James Buchanan appointed Cass secretary of state. During his tenure, however, he was overshadowed by the president's own initiatives in foreign policy and by his own increasing feebleness. He remained a loyal party man, however, and in 1860 went with the administration in supporting John C. Breckinridge as the Democratic candidate over Stephen A. Douglas. Subsequently, however, Cass argued with Buchanan over the defense of American military posts in the face of southern secession. Taking his traditional strong nationalist position, he pushed hard to reinforce the beleaguered forts at Charleston and Pensacola and resigned in disgust over Buchanan's hesitations about doing so--a resignation the latter accepted with alacrity. He returned to Michigan to spend the rest of his life, dying at his home in Detroit.