Complex sentence: Velleius Paterculus, as an officer under the command of future emperor Tiberius, experienced the wars as a witness: according to his own statement, he served from the year 4 AD under the command of Tiberius, first as a cavalry prefect.
Split into simpler sentences: Velleius Paterculus, as an officer under the command of Tiberius, experienced the wars as a witness. According to his own statement, he served from the year 4 AD under the command of Tiberius, first as a cavalry prefect.