Several answers (on Beurling, Gleason, Gröbner, Littlewood, Rankin, Robinson, Turing, Ulam) suggest that applied work was often classified. I also heard about Vieta being his King’s cryptographer and Monge’s first work being classified.

Two quotes to illustrate that sometimes this means ***“interesting achievement”**:*

>([Notices AMS 63, p. 505](//ams.org/mathscinet-getitem?mr=3445402)): Above are excerpts from two Nash letters that the National Security Agency (NSA) declassified and made public in 2012. In these extraordinary letters sent to the agency in 1955, Nash anticipated ideas that now pervade modern cryptography and that led to the new field of complexity theory. (In the obituary for Nash that appears in this issue of the Notices, page 492, John Milnor devotes a paragraph to these letters.)

and sometimes apparently not:
 
>([Mac Lane 1976, p. 138](//ams.org/mathscinet-getitem?mr=414284)): *faute de mieux*, finds himself in New York as Director of the Applied Mathematics Group of Columbia University, instructed to hire many fresh mathematical brains to help with the research side of the war effort. One of his first acts was to hire Samuel Eilenberg—as well as Irving Kaplansky, George Mackey, Donald Ling, and many others. During the day we all worked hard at airborne fire control... [The] report (more exactly, part 2 on “Aerial Gunnery Problems,” as cited in the bibliography) was initially classified *confidential* and hence buried in the Government Archives. By now it is declassified, but hardly interesting.