In 1638, his brother Eberhard III was given back certain parts of the lost duchy of Württemberg by Ferdinand III, Holy Roman Emperor while Frederick was drawn into war service. After the Peace of Westphalia, which led to the full restoration of Württemberg, Eberhard entered into a Fürstbrüderlicher Vergleich – a mutual agreement made between ducal brothers. Duke Eberhard III left his brother Frederick possession of Neuenstadt, Möckmühl and Weinsberg, although this was without sovereignty which remained with Eberhard.