Ernestine duke of Saxony, or Saxe-Coburg-Eisenach, whose attempts to regain the electoral dignity, lost by his father to the rival Albertine branch of the House of Wettin, led to his capture and incarceration until his death.

On the imprisonment of his father, the former elector John Frederick the Magnanimous, John Frederick II and his brothers John William and John Frederick III the Younger ruled the remaining Ernestine lands jointly. In 1565, on John Frederick III's death, John Frederick II became sole ruler, after ceding Coburg as a separate duchy to John William. A strict Lutheran, he repeatedly purged the clergy of his realm for teachings of which he disapproved. John Frederick never forgave the Emperor for his father's loss of the electorship. Attempting to regain the dignity, he allied himself with the powerful Palatinate and the adventurer Wilhelm von Grumbach (Grumbach, Wilhelm von) (q.v.) and tried to foment an uprising. Placed under the imperial ban in 1566, John Frederick was captured at Gotha in 1567. For the next 28 years until his death he was held prisoner in various places. His wife, Elizabeth of the Palatinate, shared his captivity.