Anthony Kenny reports that, coincidentally, Descartes had once soldiered against Princess Elisabeth’s father (Frederick V) while serving in the army of Maurice of Nassau.  But that is contested by Richard Watson in his splendid biography, who says that Descartes did no actual soldiering at all.  Alexandre Koyré comments that “[t]he military career of Descartes seems to have been a failure….  In any case it did not last long.  He was not of the stuff that makes good soldiers” (Koyré’s Introduction to G.E.M. Anscombe and P.T. Geach (eds.), Descartes: Philosophical Writings (Bobbs-Merrill, 1971, p. xviii)).  Descartes’ own father once said of him that “n’est bon qu’á être relié en veau”—pretty startling, until you learn that “veau” there meant buckskin, not veal.