John Ernest III, Duke of Saxe-Weimar

Johann Ernst I, Duke of Saxe-Weimar (b. Altenburg, 21 February 1594 - d. Sankt Martin, Hungary, 6 December 1626), was a duke of Saxe-Weimar.

He was the eldest son of Johann, Duke of Saxe-Weimar and Dorothea Marie of Anhalt.

During his first years, Johann Ernst had a house teacher and yard master, Frederick of Kospoth. In 1608 he start his studies with his brother Frederick in the University of Jena. To be accompanied and supervised the two princes was appointed the later Fiel Marshall Kaspar of Teutleben and the Preceptor Frederick Hortleder. Some years later, in 1613-1614, Johann and his brother, with his guardians, take a tour for France, Great Britain and the Netherlands in order to continue his studies.

In 1615 Johann Ernst reached adulthood and take over the government of his duchy and the guardianship of his younger brothers, who still under age.

On 24 August 1617 in the Schloß Hornstein (now Wilhelmsburg), Johann Ernst created The Fruitbearing society (Fruchtbringende Gesellschaft), a German literary society. The young duke participates as initial member.

During his government, Johann Ernst promotes the reforms of Wolfgang Ratke in Köthen. Also supported in Weimar the similar school reforms of Johannes Kromayer and Johann Weidner, starting on 1618 in Jena and Weimar with the annullment of taxes decrets.

In 1620 Johann Ernst starting to served under Frederick V, Elector of the Palatinate, the famous Emperor of One Winter. After his defeat in the battle of Weißen Berg on 8 November 1620, the duke refuses submitting without conditions to the emperor. Then, in punishment, he lost his states and the guardianship of his brothers.

At firstly against the Habsburgs, he fights in the Netherlands like ride master and later, like a danish Lieutenant general of the Kavallerie in Westphalia and Lower Saxony. As such he participates in the conquest of the Schlesiens and follows afterwards he fight for the count Peter Ernst II of Mansfeld by one of his fields in Hungary.

There Johann Ernst died, at the age of 32 years, in the hungarien locate of Sankt Martin.