Southern Italy in 1112 CE showing the major states and cities. Smaller states and city-states, usually under the suzerainty or vassalage of the larger states, are not shown.
He was the only son and successor of Jordan II of Capua. According to the Lombard chronicler Falco of Benevento, he was "of delicate constitution, he could endure neither labour nor hardship."
In the final month of 1127, Pope Honorius II came to Benevento to preach a crusade against Count Roger II of Sicily in order to prevent the union of his county with the duchy of Apulia (Duke William II being recently deceased). On 30 December, Robert was installed as prince. He was quickly recruited for the endeavour by the pope, who went to Capua for the ceremony. The pope probably hoped to use Capua as a counterpoise against Apulia, as in the days of Robert's grandfather and great grandfather. Likewise, Robert may have intended to be the chief papal protector, as his ancestors had been. However, he was weak-willed and he soon fell ill and wanted out. Eventually, the coalition commenced negotiations on Roger's arrival with an army. Honorius even successfully negotiatied the independence of Capua. In 1129, however, Robert submissively surrendered suzerainty to the duke of Apulia and, the next year (on 25 December1130), he, as Roger's vassal-in-chief, laid the crown on his head at his royal coronation.
Robert fled to Pisa, where he gathered a navy and made war against Roger in Sicily, but it was a stalemate. The Pisan fleet ravaged Amalfi and took much loot. Laden with this plunder and accompanied by a papal legation, Robert went to Germany to plead for the aid of the emperor. In Spring 1137, the emperor came down with Pope Innocent II; Henry the Proud, duke of Bavaria; and a large force. They took Benevento, Bari, and Capua itself, installing Ranulf as duke of Apulia and Robert in Capua, vindicating these actions in battle. But when the emperor left Italy, Roger sacked Capua yet again. On 25 July1139, Robert and the pope were defeated in battle on the Garigliano, at Galluccio, ambushed by Roger. The pope was captured, though Robert escaped. They thereafter acknowledged him as principatus Capuae. When Alfonso died in 1144, Roger made his fourth son William prince. When Roger died in 1154, William succeeded him as king and duke, but Robert retook Capua (1155). He died soon after and William brought Capua within the kingdom of Sicily permanently.