While out for a walk this evening, I started thinking for some reason about Descartes and the Battle of White Mountain. Descartes, of course, was a mercenary soldier; he had joined and trained with the army of Prince Maurice of Nassau. Baillet, who is generally quite reliable about such things, says that he was at the Battle of White Mountain in 1620 with the Imperial troops. We have, despite occasional skepticism, no serious reason to doubt it, but there are a number of puzzles about this.
The Battle of White Mountain marked the end of what is often called the Bohemian Revolt during the Thirty Years' War. The Protestant princes of Bohemia were more or less attempting to have Bohemia secede from its union with the Holy Roman Empire for religious reasons; they chose as their king Frederick V, the Elector Palatine, in autumn of 1619. Frederick's army was smashed to pieces on November 8, 1620 by Count Tilly just outside of Prague on the plateau of Bílá Hora, which gives the battle its name. Frederick and his family had to flee to the Netherlands. It's not surprising that they'd go there; Prince Maurice of Nassau, who was a Protestant relative (his uncle, I believe) with whom he was on good terms, had encouraged him to accept the crown of Bohemia and had financed and supported them with troops.
And therein is the first puzzle. Descartes, while Catholic, had been a soldier in the army of Prince Maurice. There is good reason to think he had left for a while, but what was he doing with the Imperial troops, given that Prince Maurice was on the other side? We don't know. Descartes never mentions anything about it in any of his surviving works; he tells very little about any of his actual life as a soldier. But Baillet is quite clear that Descartes was with the troops of the Holy Roman Empire at this time; he tells us that Descartes joined them in January of 1619 and that he continued on with them after White Mountain through the campaign into Moravia until he gave up being a soldier in the summer of 1621 -- after which he returned to the Netherlands.
Interestingly, we have no reason to think that he fought in any battles; Baillet doesn't say that he fought at the Battle of White Mountain, as you might expect, but simply that he was there and observed it. He does not tell us what that involved. Indeed, some of what he says suggests to me that he may not have known himself.
A. C. Grayling suggests that he was spying -- in particular, that he had been spying on the Dutch. This would make some sense of the limited evidence, but it's also the case that Descartes being a spy is Grayling's solution for almost every mystery in Descartes's life, so one is wary of falling back on unprovable espionage as a magical key.
Descartes, of course, would later meet (briefly) and then have a famous correspondence with Princess Elisabeth of Bohemia, the daughter of Frederick V. Such a turn of history is so perfect it is difficult to avoid imagining some connection between the two events; in a novel an author would certainly attempt to make such a connection, and if he did not, the reader would tend to do it for him. But such things may be entirely in the mind. The fact of the matter is, we don't know enough about Descartes's military career to say much about how it affected him, beyond the very tiny amount he says himself and the obvious fact that he seems to have used soldiering as a means for visiting places of scientific and philosophical interest. We don't know what he did at White Mountain, when he 'observed' the battle as part of the army that crushed Elisabeth's father's rule and forced her family into exile. We don't know if either of them ever thought of the fact that they were indirectly connected in this way -- after all, given the nature of the Thirty Year's War, the same could be said for a very large portion of the European population. So many unknown, and apparently undiscoverable, things.