Saxe-Coburg-Eisenach was one of the short lived Ernestine duchies. It only existed between 1572 - 1596 and again from 1633 - 1638. It was a part of the Holy Roman Empire, today it is within the federal states of Bavaria and Thuringia.
The duchy was created by the Division of Erfurt in 1572 when Coburg and Eisenach were split from the Duchy of Saxony-Weimar for Johann Casimir and Johann Ernst, the sons of Johann Friedrich II, the Middle.
The two princes at this time were still children, so the Elector August of Saxony was regent until 1586. This is when Johann Casimir and Johann Ernst becgan to rule the duchy. Johann Ernst retired to his hunting lodge in Marksühl, and in 1590 the two brothers divided the country. Johann Casimir kept Saxe-Coburg, and Johann Ernst took Saxe-Eisenach. Johann Casimir died in 1633so his brother Johann Ernst inherited his possessions, again were short and Coburg and Eisenach were reunited until Johann Ernst's death in 1638.
Johann Ernst also had no children, so the Saxe-Coburg-Eisenach line died out, out the territory was divided between the other Ernestine line.