Gregory IV, pope (september 20, 827-January 11, 844), was chosen to succeed Valentine in December 827, on which occasion he recognized the supremacy of the Frankish emperor Louis the Pious in the most unequivocal manner.
Papal dependence on the Holy Roman Emperor loosened through the quarrels of Louis I the Pious and his sons, the future Lothair I, Pepin and Louis the German. On the sons' rebellion against their father, Gregory supported Lothair, hoping his intervention would promote peace, but in practice this action annoyed the Frankish bishops. Gregory's response was to insist upon the primacy of St Peter's successor, the papacy being superior to the Emperor.
The two armies, of Louis and his sons, met at Rotfeld, near Colmar, in the summer of 833. The sons persuaded Gregory to go to Louis' camp to negotiate, but he then found he had been duped by Lothair. Louis was deserted by his supporters and was forced to surrender unconditionally, and was deposed and humiliated. This sequence of events is known as the Campus Mendacii or "field of lies." Louis was subsequently restored, and after his death Gregory made unsuccessful attempts to mediate in the conflict that ensued between the brothers.
He is also known for his appointment of Ausgar for archbishop of Hamburg and Bremen, and a missionary delegate for north and east parts of Europe. He also fortified the port of Ostia against the attacks of Saracens.