As a younger son, Rupert was originally destined for a career in the
clergy. Gerlach I abdicated in 1344, in favour of his sons, except he kept
Sonnenberg Castle, where he lived. This castle was to be inherited by Kraft, Rupert's eldest brother. However, Kraft fell in battle while fighting on the French side in the
Battle of Poitiers in 1356. It was then decided that Rupert would inherit Sonnenberg. Rupert's half-brother from his father's first marriage ruled their father's possessions jointly until 1355, and then divided it: •
Adolph I inherited Nassau-Wiesbaden-Idstein (this line died out in the male line in 1605) •
John I inherited Nassau-Weilburg (this line died out in the male line in 1912) • Rupert later inherited Nassau-Sonnenberg. From 1355, Rupert was bailiff at
Amöneburg Castle for his half-brother
Gerlach, who was Archbishop of
Mainz. In 1361, Gerlach I died and Rupert inherited Nassau-Sonnenberg. Gerlach's widow Irmengard retired to
Liebenau monastery, near
Worms, where she became a
Dominican nun. In 1367, Rupert pledged some of his territory to his half-brother Adolph I, who immediately pledged it to back to Rupert and Anna (Ruper's wife) and Irmengard (Rupert's mother and Adolph's step-mother). The reasons for this remarkable maneuver are unclear. In 1369, Rupert started a
feud against
John I of Nassau-Siegen, about some
fief John I held from
Hesse. Rupert claimed these fiefs, and joined the
Star Covenant, an alliance of knights against Landgrave
Henry II of Hesse. In 1370, his half-brother Adolph I died. In 1371, his mother, Irmengard, died, as did his half-brothers John I and Gerlach. From 1372 to 1374, Rupert fought another feud against John I of Nassau-Siegen. He managed to keep his Sonnenberg Castle intact, whereas the town of
Nassau was so badly damaged during the fighting that it was uninhabitable for a while. Rupert forced John to hand over the town of
Hadamar to Anna. They divided the territory. In 1381, King
Wenceslaus appointed Rupert governor of the
Wetterau. This triggered a second feud against John I of Nassau-Siegen, which lasted until 1382. In 1382, Rupert and the Counts of Solms-Burgsolms expanded
Greifenstein Castle and added the characteristic twin towers. Also in 1382, yet another feud against John I of Nassau-Siegen broke out, which lasted until 1385. After the feud he fought in the
Hattstein War on the side of the cities in the Rhine/Wetterau region, against the high nobility in the area. Rupert died in 1390 and was buried in
Kirchheimbolanden. As he had no children, Nassau-Sonnenberg was inherited by
Philipp I of Nassau-Weilburg. == Marriage ==