William Ernest, Duke of Saxe-Weimar

Wilhelm Ernst, Duke of Saxe-Weimar (19 October 1662 - 26 August 1728), was a duke of Saxe-Weimar.

He was born in Weimar, the eldest son of Johann Ernst II, Duke of Saxe-Weimar and Christine Elisabeth of Holstein-Sonderburg.

When his father died in 1683, he succeeded him as Duke; however, was compelled to co-rule with his younger brother Johann Ernst III.

Because Johann Ernst III was alcoholic, Wilhelm Ernst took the full control of the government of the duchy and let to Johann Ernst the nominal title of co-duke ("Mitherr") until his death in 1707. After the death of his brother he made his son Ernst August I co-duke but, like his father, without any real power.

In order to secure the family lands, six months after the death of his father (2 November 1683), Wilhelm Ernst married, in Eisenach, with Charlotte Marie of Saxe-Jena, his cousin and eldest surviving daughter of his uncle Bernhard. At that time, the guardian of Charlotte and his younger brother, the duke Johann Wilhelm of Saxe-Jena, was the duke Johann Georg I of Saxe-Eisenach, their only surviving uncle. When he died in 1686, the guardianship of the duke of Saxe-Jena was taken by Wilhelm Ernst, his cousin and brother-in-law.

Wilhelm Ernst was an strict Lutheran; during the training of his soldiers, he command to only can join under his service the men who could read and comment with him the contents of Lutheran lecture, be repeated in the winter around eight o'clock and in the summer at nine o'clock. This particular way of doctrine was totally supported by Johann Sebastian Bach. The famous composer worked since 1708 as Master organist and concert in Weimar. When Johann Samuel Drese died in 1716, Bach solicited his post of Master of the Chapel, but Wilhelm Ernst appointed the incompetent son of Drese for the post; furious, Bach solicited his dismissal from the Duke's service. Annoyed of these reticence of the musician, Wilhelm Ernst let Bach four weeks in a fortress detention, before he accepted his dismissal. This procedure led to the fact that William Ernst in all Bach biographies received a place as authoritarian ruler.

On 23 August 1690 Wilhelm Ernst and Charlotte Marie were divorced after seven years of childless union. Two months later, on 4 November, the duke Johann Wilhelm of Saxe-Jena (Charlotte Marie's brother), died without heirs, and Wilhelm Ernst take part of his duchy after a divisionary treaty with the branch of Saxe-Eisenach.

Wilhelm Ernst never remarried and died at Weimar without heirs; his nephew Ernst August I became his successor.