Simplify the following text for a Grade 8 native English-speaking person.
Original: After the Division of Erfurt in 1572 , Coburg was part of the Duchy of Saxe-Coburg-Eisenach , ruled by the Ernestine duke John Casimir jointly with his brother John Ernest .
Simplified: It was part of the Duchy of Saxe-Coburg-Eisenach after the Division of Erfurt in 1572 .