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, to whom he had to surrender on 15 August of the same year. In 1637, he supervised the Hessian occupation of
Oldersum in
East Frisia. Following the ascension of Countess
Amalie Elisabeth to the throne in September 1637, Geyso gains a more prominent role in the Langravate's
military campaigns against the rival
Landgraviate of Hesse-Darmstadt. In the autumn of 1641, he commanded the
defense of
Dorsten finally handing it over to Imperial troops on favorable conditions. In 1644, he was summoned by Swedish field marshal
Lennart Torstensson who requested his assistance with the encircling of Imperialist
Matthias Gallas' army which at the time operated in the vicinity of
Magdeburg. Geyso's 2,300 men took part in grueling march through flooded areas, arriving in time to witness the disintegration of the Imperialist army. On 3 August 1645, the courage of the Hessians contributed to the victory at the
Battle of Nördlingen (1645). ==Return to Hesse==