Geyso was born to a family of commoners in 1593 in Borken, Hesse. At an early age he was sent to a military academy in the Dutch Republic by Maurice, Landgrave of Hesse-Kassel, where he studied the art of war. He then joined a Swedish Banner which fought in the Ingrian War and the Polish–Swedish War (1600–29) against Russia and Poland respectively. He then returned to the Holy Roman Empire, entering the service of rebellious Bohemian Estates who sparked the Thirty Years' War through the Bohemian Revolt. He fought in the Battle of White Mountain, where he commanded a Fähnlein of infantry, while being in the rank of a captain. After the defeat of the Bohemians he continued to fight for the Protestant Union first under Ernst von Mansfeld, then Bernard of Saxe-Weimar as Rittmeister, finally entering the service of Christian IV of Denmark after the latter intervened into the war. In 1628, he fought as a colonel at the Battle of Lutter, where the Danes were defeated. Whereupon Countess Juliane of Hesse-Kassel requested him to return into his homeland, to which he complied. In 1630, Sweden launched its own intervention into the war, reinvigorating the seditious Protestants in the empire. Geyso was appointed quartermaster general, assisting William V, Landgrave of Hesse-Kassel in reorganizing his troops. His next assignment on the field came in 1636 when he defended Paderborn from the combined forces of Johann von Götzen and Gottfried Huyn von Geleen, whom he repulsed on 15 August of the same year. In 1637,