Complex sentence: 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.
Split into simpler sentences: The Eastern Roman emperor Justinian, took this as a pretext to interfere in Vandal affairs. After he secured his eastern frontier with Sassanid Persia in 532, he began preparing an expedition under general Belisarius, whose scribe Procopius wrote the main historical narrative of the war.