 Its territory was part of the modern states of Bavaria and Thuringia.The duchy was created by the Division of Erfurt in 1572 which implemented a decision of the Diet of Speyer in 1570 to separate Coburg and Eisenach from the Duchy of Saxe-Weimar and give them to John Casimir and John Ernest, the two sons of John Frederick II.However, because the two princes were still minors at the time, the country was at first ruled by Elector August of Saxony.