At the end of his life, his ambitions, more and more grandiose, lay in the direction of the Eastern Empire. The repeated excommunications of Robert by Pope Gregory VII, in 1074, 1075 and in 1078, after Robert’s attempt to seize the principality of Benevento, led to a sudden deterioration in the relations between the Normans and the papacy. During the investiture quarrels, the Pope, in serious conflict with Emperor Henry IV, who himself had been excommunicated, could not manage without Norman support. Thus in 1080, in Ceprano, Robert Guiscard, whom the Pope had called ‘a small humble Norman’, a few years earlier, solemnly swore allegiance to the Papal power, who would soon call for his help against the invading German emperor in Rome. His alliance, both matrimonially and politically, was sought after, as much by the German Emperor as the Byzantine emperor. Emperor Michael VII had suggested a military alliance and the marriage of the Emperor’s brother to Guiscard’s daughter Helen and also bestowed high Byzantine honours on Guiscard’s family. Taking advantage of a period of political anarchy and troubles in Byzantium, which had lasted since 1076, Guiscard, ostensibly seeking to restore Michael VII, who had been overthrown in favour of Nicephorous Botanoiates in 1078, and with his daughter confined to a convent, decided to attack Byzantium. To guarantee Apulia against attack from the new rulers of Byzantium, Robert wanted the territories on the Adriatic coast of the Balkan Peninsula, and he began to build a large navy.
Crisis within the Byzantine Empire in the late 1070s enabled Robert Guiscard to undertake an audacious enterprise against a weakened state. In 1081 he, assisted by his son Bohemond crossed the Adriatic Sea with a considerable navy and invaded mainland Greece. The first campaign in 1081-1082 saw a series of victories on the Dalmatian coast and in Macedonia. The Norman leaders benefited from papal support for their success at Durazzo in 1081 whereas at Hastings in 1066 they had the papal banner: William I received his from Pope Alexander II and Robert Guiscard from Pope Gregory VII. Initially, he had some success but a combination of Norman revolts in Apulia and Alexius Comnenus becoming emperor in 1081 compelled him to return to Italy to reassert his authority in April 1082. In addition, the German emperor Henry IV invaded Italy with a considerable force in 1083. Pope Gregory VII menaced by the German army and by pro-German supporters in Rome appealed to Robert Guiscard for help. Having defeated the Norman rebels in Apulia, Robert marched on Rome and freed the pope. Returning, in autumn 1084, the second campaign advanced favourably. However, when he arrived on Cephalonia, off Epirus, Robert died of dysentery on 17th July 1085 at Cape Antheras.
Physically attractive, endowed with an acute and unscrupulous intelligence, a brilliant strategist and competent statesman, Robert had begun to organise a state composed of diverse ethnic groups: Latin and Germanic in Lombard territories and Greek in the Byzantine domains. The new political structure was built on a monarchical-feudal framework characteristic of the time. However, Robert controlled it by using his ducal power to create a powerful and prosperous state. The other base on which he built was Roman Catholicism, the religion of the conquerors and most of the conquered, which he used to reconcile the subjected peoples. An extremely religious man, Robert was distrustful of the Greek clergy because of their ties with Byzantium. On the other hand, his generosity toward the Latin Church was considerable. He endowed it with territories and clerical immunities in order to tie it firmly to the feudal system. Splendid cathedrals and Benedictine abbeys were built in the hope that they would consolidate and diffuse Latin language and culture among the heterogeneous people and tie them into a new, unified state. Robert was kept from realising this political vision only by his death.