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 Darmstadt 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).