In 1854, the prince was raised to the rank of a field-marshal and made governor of the federal fortress of Mainz. In 1857 Frederick William IV suffered a stroke and became mentally disabled for the rest of his life. In January 1858, William became Prince Regent for his brother, initially only temporarily but after October on a permanent basis. Against the advice of his brother, William swore an oath of office on the Prussian constitution and promised to preserve it "solid and inviolable". William appointed a liberal, Karl Anton von Hohenzollern-Sigmaringen, as Minister President and thus initiated what became known as the "New Era" in Prussia, although there were conflicts between William and the liberal majority in the Landtag on matters of reforming the armed forces.