Complex sentence: After the death of his father in 1516, Joachim and his older brothers John V and George III became in the new rulers of the Anhalt-Dessau; during the first years of their reign, his mother acted as regent.
Split into simpler sentences: After the death of his father in 1516, Joachim and his older brothers John V and George III became the new rulers of the Anhalt-Dessau. During the first years of their reign, their mother acted as regent.