He fought in Pomerania, Bohemia and Saxony (surprise of Wolmirstadt, battles of Wittstock and Chemnitz), and in 1639 he was taken prisoner at Melnik and detained for two and a half years in Stettin and Weimar. In captivity he studied military science, and also geometry by the way of Euclid, history of Tacitus, and Vitruvius' architecture, all the while planning his great work on war.