In 1628, having gained significant experience in warfare he returned to his native Hesse-Kassel which he served until the end of the Thirty Years' War, reaching the rank of commander in chief of the Langraviate's forces and becoming ennobled.

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.

[2] 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 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.

Darmstadt then hired Ernst Albrecht von Eberstein as their new commander in chief, his counter offensive liberated all lands previously lost to Kassel apart from Marburg.

On 14 July, he defeated Lamboy at the Battle of Wevelinghoven,[3] skirmishes between the two continued until the end of the war denying Imperial troops of badly needed reinforcements.