Marcus Claudius Marcellus was the eldest son of Gaius Claudius Marcellus and Octavia the Younger, sister of Augustus. He was Augustus' nephew and closest male relative, and began to enjoy an accelerated political career as a result. He was educated with his cousin Tiberius and traveled with him to Hispania where they served under Augustus in the Cantabrian Wars. In 25 BC he returned to Rome where he married his cousin Julia, who was the emperor's daughter. Marcellus and Augustus' general Marcus Vipsanius Agrippa were the two popular choices as heir to the empire. According to Suetonius, this put Agrippa at odds with Marcellus, and is the reason why Agrippa traveled away from Rome to Mytilene in 23 BC.