Career
John reigned from 1308 to 1311 as Landgrave in Lower Hesse. This led to armed clashes which lasted until Henry's death. At Mechtild's insistence, John was made co-ruler in 1296.
The inheritance was eventually divided after Henry died in 1308. John received Lower Hesse with the capital Kassel and the imperial fiefs. His half-brother Otto I received the Land of the Lahn, the later Upper Hesse, with the capital Marburg.
Otto's part did not include imperial fiefs. Hesse had mortgaged Gudensberg to the Duchy of Brunswick-Lüneburg. In 1309, John conquered the city and forced Duke Albert II of Brunswick-Göttingen to accept his repayment.
Emperor Henry VII appointed him protector of the free imperial cities of Mühlhausen, Nordhausen and Goslar. Margrave Frederick I saw this as an intrusion into his Landgraviate of Thuringia and took up arms to stop the intrusion. The fight went badly for John and he had to retire to Kassel to recover.
John died of the plague on 14 February 1311 in Kassel. He was buried in the Ahnaberg monastery. No further hostitlites between John and Frederick took place and John's plan to create a new fortified city on the Thuringian side of the river Fulda was never implemented.
After John's death, Lower Hesse fell to Otto I.
In 1306, John married Adelaide of Brunswick-Lüneburg, the daughter of Duke Albert II of Brunswick-Göttingen. John and Adelaide had a daughter, Elisabeth (d 1339).
