Under the reorganization of Francia by Charles Martel the dux et princeps Francorum were the commanders of the armies of the kingdom, in addition to their administrative duties as mayor of the palace, and specifically commander of the standing guard which Martel had begun maintaining year-round since Toulouse in 721.
Upon their assumption, Pepin and Carloman, who had not proved themselves in battle in defense of the realm as their father had, installed Childeric III as king, even though Martel had left the throne vacant since the death of Theuderic IV. Childeric had the title of king, but he was considered weak. As time passed, and his brother bowed out of the picture, Pepin became discontent with the royal power being with Childeric.
At the time of Carloman's retirement, Grifo escaped his imprisonment and fled to Duke Odilo of Bavaria, who was married to Hiltrude, Pepin's sister. Odilo was forced by Pepin to acknowledge Frankish overlordship, but died soon after (January 18 748). Pepin invaded Bavaria and installed Tassilo III as duke under Frankish overlordship.
Pepin's first major act was to go to war against the Lombard king Aistulf, who had a policy of expansion into the ducatus Romanum, as a partial repayment for papal support in his quest for the crown. Victorious, he forced the Lombard king to return property seized from the Church and confirmed the papacy in possession of Ravenna and the Pentapolis, the so-called Donation of Pepin whereby the Papal States was founded. In 759, he drove the Saracens out of Gaul with the capture of Narbonne and then consolidated his power further by integrating Aquitaine into the kingdom. In taking Narbonne, and formally annexing Aquitaine (whose status was always dependent on the strength of her suzerains), he completed the work of his father save for one last task: fully subduing the Saxons. He was preparing for war against them when his health began to fail, and thus, this final task was left for his son, the great Charlemagne.
Historical opinion often seems to regard him as the lesser son and lesser father of two greater men, though a great man in his own right. He continued to build up the heavy cavalry which his father had begun. He maintained the standing army that his father had found necessary to protect the realm and form the core of its full army in wartime. He not only maintained his father's policy of containing the Moors, he drove them over and across the Pyrenees with the capture of Narbonne. He continued his father's expansion of the Frankish church (missionary work in Germany and Scandinavia) and the infrastructure (feudalism) that would prove the backbone of medieval Europe.
His rule, while not as great as either his father's or son's, was historically important and of great benefit to the Franks as a people. It can certainly be argued that Pepin's assumption of the crown, and the title of Patrician of Rome, were harbingers of his son's imperial coronation which is usually seen as the founding of the Holy Roman Empire. He certainly made the Carolingians de jure what his father had made them de facto—the ruling dynasty of the Franks and the foremost power of Europe. While not known as a great general, he was undefeated during his lifetime.