Henry IV’s minority also gave elbowroom to the ambitions and hatreds of the lay magnates. The feeble regency of his mother, the pious Agnes of Poitou, faltered before the throng of princes, who respected only authority and power greater than their own. The influence of the higher clergy at the court of Henry III and the renewed flow of grants to the church had estranged these princes from the empire. It is likely also that these eternally belligerent men lagged behind the prelates in the development of their agrarian resources. The prelates had a vested interest in peace, and under royal protection they improved and enlarged their estates by turning forests into arable land and also by offering better terms to freemen in search of a lord. The bishops’ market and toll privileges brought them revenues in money, which many of the lay princes lacked. So far, however, the princes’ military power, their chief asset, had remained unchallenged. Now, for the first time, they also had to face rivals within their own sphere of action. Henry III and the young Henry IV began to rely on advisers and fighting men drawn from a lower tier of the social order—the poorer, freeborn nobility of Swabia and, above all, the class of unfree knights, known as ministeriales. These knights had first become important as administrators and soldiers on the estates of the church early in the 11th century. Their status and that of their fiefs was fixed by seignorial ordinances, and they could be relied on and commanded, unlike the free vassals of bishops and abbots. Beginning with Conrad II, the Salian kings used ministeriales to administer their demesne, as household officers at court, and as garrisons for their castles. They formed a small army, which the crown could mobilize without having to appeal to the lay princes, whose ill will and antipathy toward the government of the Reich grew apace with their exclusion from it.

Having come of age, Henry IV used petty southern German nobles and his ministeriales to recover some of the crown lands and rights that the lay princes and certain prelates had acquired during his minority, particularly in Saxony. His recovery operations went further, however, and a great belt of lands from the northern slopes of the Harz Mountains to the Thuringian Forest was secured and fortified under the supervision of his knights to form a royal territory, where the king and his court could reside. The southern German magnates were thus kept at a distance when Henry and his advisers struck at neighboring Saxon princes such as Otto of Northeim and at the Billung family.

The storm broke in 1073. A group of Saxon nobles and prelates and the free peasantry of Eastphalia, who had borne the brunt of statute labor in the building of the royal strongholds, revolted against the regime of Henry’s Frankish and Swabian officials. To overcome this startling combination and to save his fortresses, the king needed the military strength of the southern German princes Rudolf of Rheinfelden, duke of Swabia; Welf IV, duke (as Welf I) of Bavaria; and Berthold of Zähringen, duke of Carinthia. Suspicious and hostile at heart, they took the field for him only when the Eastphalian peasantry committed outrages that shocked aristocrats everywhere. Their forces enabled Henry to defeat the Saxon rebellion at Homburg near Langensalza in June 1075. But, when the life-and-death struggle with Rome opened only half a year later, the southern German malcontents deserted Henry and, together with the Saxons and a handful of bishops, entered into an alliance with Gregory VII. Few of them at this time were converted to papal reform doctrines, but Gregory’s daring measures against the king gave them a chance to come to terms with one another and to justify a general revolt.