In the autumn of 1236, Frederick conquered and deposed Frederick the Quarrelsome, Duke of Austria, and made Vienna a free Imperial city. A Diet was held there, at which his second son, Konrad, then nine years old, was accepted as king of Germany. This choice was confirmed by another Diet, held the following year at Speyer. The Emperor now left Germany, never to return. This brief visit, of a little more than a year, was the only interruption in his thirty years of absence; but it revived his great personal influence over princes and people, it was marked by the full recognition of his authority, and it contributed, in combination with his struggle against the power of Rome which followed, to impress upon his reign a more splendid and successful character than his acts deserved. Although the remainder of his history belongs to Italy, it was not without importance for the later fortunes of Germany, and must therefore be briefly stated. [Sidenote: 1237.] On returning to Italy, Frederick found himself involved in new difficulties with the independent cities. He was supported by his son-in-law, Ezzelin, and a large army from Naples and Sicily, composed chiefly of Saracens. With this force he won such a victory at Cortenuovo, that even Milan offered to yield, under hard conditions. Then Frederick II. made the same mistake as his grandfather, Barbarossa, in similar circumstances. He demanded a complete and unconditional surrender, which so aroused the fear and excited the hate of the Lombards, that they united in a new and desperate resistance, which he was unable to crush. Gregory IX., who claimed for the Church the Island of Sardinia, which Frederick had given as a kingdom to his son Enzio, hurled a new excommunication against the Emperor, and the fiercest of all the quarrels between the two powers now began to rage. The Pope, in a proclamation, asserted of Frederick: "This pestilential king declares that the world has been deceived by three impostors, Moses, Mohammed and Christ, the two former of whom died honorably, but the last shamefully, upon the cross." He further styled the Emperor, "that beast of Revelations which came out of the sea, which now destroys everything with its claws and iron teeth, and, assisted by the heretics, arises against Christ, in order to drive his name out of the world." Frederick, in an answer which was sent to all the kings and princes of Christendom, wrote: "The Apostolic and Athanasian Creeds are mine; Moses I consider a friend of God, and Mohammed an arch-impostor." He described the Pope as "that horse in Revelations, from which, as it is written, issued another horse, and he that sat upon him took away the peace of the world, so that the living destroyed each other," and named him further: "the second Balaam, the great dragon, yea, even the Antichrist." [Sidenote: 1241. CAPTURE OF THE POPE'S COUNCIL.]