Joseph Holt is most famous for having served as chief prosecutor in the Lincoln assassination trials in his capacity as Judge Advocate General. (If you’ve seen The Conspirator, you might not have the most favorable impression of him.) Holt was also a major figure in the last-minute reorganization of Buchanan’s cabinet during the secession crisis. Jeremiah Black and Edwin Stanton were brought into the cabinet as constitutional Unionists in late December 1860. John Floyd of Virginia resigned his Secretary of War post on December 29, and Buchanan found a suitable replacement in Postmaster General Joseph Holt, a Unionist from Kentucky.
The beleaguered Buchanan wrote out every letter of Holt’s nomination himself, an extreme rarity in any period. The expression “Secretary of War of the United States” is a bit odd by modern standards, but I suppose it got the man nominated.