William the Victorious, Duke of Brunswick-Lüneburg

William KG (c. 1392 – 1482), called the Victorious (German: Wilhelm der Siegreiche), a member of the House of Welf, was Duke of Brunswick-Lüneburg. He was reigning prince of Lüneburg from 1416 to 1428 and of Brunswick-Wolfenbüttel from 1428 to 1432, counted either as William III or William IV. From 1432 he ruled over the newly established Principality of Calenberg, from 1463 also over the Principality of Göttingen. In 1473 he stepped down in favour of his sons, to assume the rule in Brunswick-Wolfenbüttel.

William was the eldest son of the Brunswick duke Henry the Mild and his consort Sophia, daughter of the Griffin duke Wartislaw IV of Pomerania. Upon his father's death in 1416, he inherited the Principality of Lüneburg-Celle which he ruled jointly with his younger brother Henry the Peaceful. William turned out to be an energetic ruler; he soon entered into numerous feuds with neighbouring princes such as the Archbishop of Bremen and the Bishop of Hildesheim, and supported the Counts of Schauenburg and Holstein in their fight against King Eric of Denmark. He also fought with Margrave Frederick of Meissen defeating the Hussite forces in the 1421 Battle of Brüx.

In 1428 William and Henry swapped Lüneburg with their uncle Duke Bernard I, for the Principality of Brunswick-Wolfenbüttel and the later Calenberg territory stretching from the Deister hill range to the Leine river. However, while again on a campaign in 1432, William was deposed by his brother. After fierce fratricidal warfare, he retained only the western part of Wolfenbüttel (to the west of the River Leine and separated from the rest of the Brunswick territory by the Prince-Bishopric of Hildesheim), which was to become known as the Principality of Calenberg, named after Williams residence at Calenberg Castle.

When the Brunswick princes of Göttingen became extinct with the death of Duke Otto the One-Eyed in 1463, William was able to take over their principality. After Henry the Peaceful died without sons in 1473, William had control of both parts of Brunswick-Wolfenbüttel again; he ceded Calenberg and Göttingen to his sons Frederick III and William the Younger.

In Berlin between 30 May/6 June 1423, William married firstly Cecilia (born c. 1405 – died 4 January 1449), daughter of the Hohenzollern elector Frederick I of Brandenburg. They had two sons:

• Frederick "the Turbulent" (c. 1424–1495)
• William "the Younger" (c. 1425–1503)

In 1466, William married secondly Matilda of Holstein-Schauenburg, daughter of Count Otto II of Schauenburg-Pinneberg, and widow of Duke Bernard II of Brunswick-Lüneburg. She died in Neustadt am Rübenberge on 22 July 1468, two days after giving birth a short-lived son, Otto (born 20 July 1468 – died 1471).