Desiderius, born in Brescia, was originally a royal officer, the dux Langobardorum et comes stabuli, "constable and duke of the Lombards," an office apparently similar to the contemporaneous Frankish office of dux Francorum.

King Aistulf had made him duke of Istria and Tuscany and he becomes king after the death of Aistulf in 756.

At this time, Aistulf's predecessor, Ratchis, leaves his monastic retreat of Montecassino and tries to seize the kingdom, but Desiderius puts his revolt down quickly with the support of Pope Stephen II.

At his coronation, Desiderius promises to restore many lost papal towns to the Holy See, in return for the papacy's endorsement of his claim.

Desiderius—seeking, like his predecessors, to extend the Lombard power in Italy—comes into collision with the papacy and the southern duchies.

In the same year Desiderius associates to his kingdom the son of Adelchis, Alboin, the duchy of Benevento and Liutprand, that of Spoleto are coaxed by Pope Stephen to commend themselves to the Franks and thus separate themselves again from monarchy.

They then place themselves under the protection of Pepin.

Paul, the brother of Pope Stephen II, who dies on April 26, 757, prevails over a faction that wants to place the Archdeacon Theophylact on the Holy See and is chosen his brother's successor by the majority, who wish a continuation of the late pope's policy.

The new pope's reign will be dominated by relations with the Frankish and Lombard kings and with the Eastern emperor.

He adopts an independent tone in informing the imperial Exarch in Ravenna of his election, but writes to Pepin the Younger that the Frankish alliance should be maintained unimpaired, being possibly forced to this course by King Desiderius of the Lombards.

Duke Liutprand of Benevento attains his majority and rebels in 758.

Desiderius defeats him and grants his duchy to one Arechis, tying the duchy more closely to Pavia than it has been since Grimoald's time.

Desiderius deposes Alboin of Spoleto in this same year, and exercises himself the ducal powers here.

The Lombards also hold the cities of Imola, …

…Osimo, …

…Bologna, and …

…Ancona, which are claimed by Rome.

In 758 also, Desiderius visits Rome and compels Paul to write to Pepin asking him to concede all the Lombard Direct claims except that to Imola; another letter of exactly opposite tenor is sent by the same messenger.

Pepin finds it advisable to maintain good relations with Desiderius, and Paul apparently accomplishes little by his double-dealing.

Later, however, Pepin will give the pope some support and act as arbiter between the Roman and Lombard claims.