The sudden disappearance in Gaeta of the co-hypati Constantine and Marinus I after 866 strongly suggests that perhaps Docibilis' assumption of power had been violent.

He is first cited as a prefecturius and then as hypatus from 877, when he followed his predecessor's example and associated his son John with him.

In his first years in office, he was faced with the Saracen menace and he fell into their hands.

After being liberated by Amalfi, he made peace with the Saracens and had been excommunicated by Pope John VIII.

In 876, the pope was down in the Mezzogiorno recruiting the princes of Capua and Salerno for the war with the Moslems.

Docibilis had met the pope at Traetto, but could not come to terms.

The pope had then interfered in the Capuan succession on the death (879) of Landulf II to impose Pandenulf over Lando in return for Pandenulf attacking Docibilis.

Formia had been captured and Docibilis had called up some Saracen mercenaries from Agropoli.

He met the pope at Gaeta itself and made peace.

Together they had besieged the Saracen fortress on the Garigliano.

After the pope's death, Docibilis had turned around and attacked Capua, according to Erchempert, with Saracen mercenaries in 900 and 903.

He now begins to turn and form alliances with the Lombard rulers, marrying his daughter Megalu to Rodgipert of Aquino and Euphemia to the prefect of Naples.

John I of Gaeta had begun his rule as an associate of his father Docibilis I from either 867, right after his father's violent takeover, or 877, when he is first mentioned as co-regent.

In that year he had received the honorific patrikios from Emperor Constantine VII.

His father disappears from the annals in 906, but he is only confirmed dead in 914.

Nonetheless, the intervening period has been John's.

He has recognized his brother Anatolio as duke of Terracina and sold the castle of Dragoncello to his other brothers.

He has begun to reverse the policy of his father of alliance with the Saracens and war with his Lombard and Greek neighbors.

He marries his daughters off strategically: Gemma to the Sorrentine prefect Marinus, Maru to the Salernitan nobleman Guaifer, and Matrona to Campolo, of an important Gaetan family.

Probably from the earliest, in 906, John had associated his own son Docibilis in a co-regency, certainly by 914.

The forces of the new imperial strategos of Bari, Nicholas Picingli, join those of various other south Italian princes of Lombard or Greek extraction: Landulf I of Benevento, John I and his son Docibilis II of Gaeta, Gregory IV and John II of Naples, and Guaimar II of Salerno.

Meanwhile, Berengar brings with him troops from the northern parts of Italy, and the campaign is coordinated by John X, who takes to the field in person, alongside Duke Alberic I of Spoleto.

After some preliminary engagements at Campo Baccano and at Trevi, the Saracens are driven to their stronghold on the Garigliano.

Here, at the Battle of Garigliano, the allies proceed to lay siege to them for three months, at the end of which the Saracens burn their houses and attempt to burst out of the encirclement.

With Pope John leading the way, all are eventually caught and killed, achieving a great victory and removing the ongoing Saracen threat from the Italian mainland.

John then confirms the granting of Traetto to John I of Gaeta, as a reward for abandoning his Saracen allies.

John of Gaeta can expand his duchy to the Garigliano, and receives the title of patricius from Constantinople, leading his family to proclaim themselves "dukes", while Alberic's prestige after the victorious battle grants him a preeminent role in the future history of Rome.