Around 1079, tensions arose between Croatia and the neighboring Holy Roman Empire, whose duke "Vecelin" (a servant of emperor Henry IV) was preparing for an attack on Croatia from his estates in Istria. The pope intervened on behalf of Zvonimir, urging Vecelin to make any complaint to him directly regarding any issues with the king. The "Annales Carinthiæ" and Chronica Hungarorum record that Zvonimir eventually invaded Carinthia to aid Hungary in war between 1079 and 1083, but this is disputed. Demetrius Zvonimir also took a hard line against the Byzantine Empire, but, unlike Peter Krešimir IV, he was also an ally of the Normans, with whom he joined in wars against Byzantium. When Robert Guiscard, Duke of Apulia, invaded the western Balkan provinces of the empire in 1084, Zvonimir sent troops to his aid.