The Eastern Roman emperor Justinian took this as a pretext to interfere in Vandal affairs, and after he secured his eastern frontier with Sassanid Persia in 532, he began preparing an expedition under general Belisarius, whose secretary Procopius wrote the main historical narrative of the war. Justinian took advantage of, or even instigated, rebellions in the remote Vandal provinces of Sardinia and Tripolitania. These not only distracted Gelimer from the Emperor's preparations, but also weakened Vandal defences through the dispatch of the bulk of the Vandal navy and a large portion of their army under Gelimer's brother Tzazon to Sardinia.