Having made peace with Henry, count palatine of the Rhine and brother of Otto IV., and settled a dispute about the lands of the extinct family of Zähringen in the south-west of the country, Frederick left Germany in August 1220; engaged in his bitter contest with the Papacy and the Lombard cities, in ruling Sicily, and, after several real or imaginary delays, in fulfilling his crusading vow, he did not return to it for fifteen years. During this period he was represented by his son Henry, in whose name the government of Germany was carried on by the regent Engelbert, archbishop of Cologne. While Engelbert lived the country was in a fairly peaceable condition, although, thanks to the emperor's concessions, the spiritual princes were predominant, and all possible means were taken to check the growth of the towns, whose interests and aspirations were not favourable to this state of affairs. There was, moreover, a struggle between Valdemar of Denmark and some neighbouring German nobles. But after Engelbert's murder (November 1225) there was a change for the worse, and the only success which can be placed to the credit of the German arms during the next few years was the regaining of the lands ceded to Denmark in 1215, lands which included the cities of Hamburg and Lübeck. Under the rule of the new regent, Louis I., duke of Bavaria, confusion reigned supreme, and civil war prevailed in nearly every part of the country.
After the treaty of San Germano, which was made with Pope Gregory in 1230, and the consequent lull in the struggle with the Papacy, Frederick was able to devote some little attention to Germany, and in 1231 he sanctioned the great Privilege of Worms. This was a reward to the princes for their efforts in bringing about the peace, and an extension of the concessions made in 1220. The princes, now for the first time referred to officially as _domini terrae_, were given full rights of jurisdiction over their lands and all the inferior officers of justice were made subservient to them. Practically they became independent sovereigns, and to make their victory more complete serious restraints were laid upon the freedom of the towns. Before this date King Henry had begun to take a personal part in the government and was already involved in a quarrel with Otto II., duke of Bavaria. He disliked the Privilege of Worms and, favouring the towns against the princes, his policy was diametrically opposed to that of the emperor; however, in 1232 he went to Italy and promised to obey his father's commands. But in 1234, at a time of great and increasing disorder in Germany, he rebelled; he appealed publicly to the princes for support, gained some followers, especially in his own duchy of Swabia, and made an alliance with the Lombard cities. Confident of his strength Frederick entered Germany with a few attendants in the middle of 1235, and his presence had the anticipated effect of quelling the insurrection; Henry was sent a prisoner to Italy and disappeared from history. Then, in August 1235, amid surroundings of great splendour, the emperor held a diet at Mainz, which was attended by a large number of princes. This diet is very important in the legal history of Germany, because here was issued that great "land peace" (_Landfrieden_) which became the model for all subsequent enactments of the kind. By it private war was declared unlawful, except in cases where justice could not be obtained; a chief justiciar was appointed for the Empire; all tolls and mints erected since the death of Henry VI. were to be removed; and other provisions dealt with the maintenance of order.