One of the coins in my posting about the Aqua Marcia featured on the obverse a portrait of Ancus Marcius. The coin is reposted above and I just wanted expand a little on who he was.
He was, traditionally, the fourth king of Rome, 642-616 BC; Romulus, Numa Pompilius (the maternal grandfather of Ancus Marcius) and Tullus Hostilius preceeding him.
The concept of a 'just' war is ascribed to Marcius. In a ritual, still practiced in a modified fashion in the second century AD, war was formally declared on another country only after a Roman priest had visited the territory, calling on each person he met and Jupiter himself, to witness that satisfaction was demanded in the name of religion and justice.
He also built an early prison, which was founded in around 625 BC, and was used to hold people until it was decided what to do with them or unishments they should serve. Before this time a popular punishment was to simply exile people.
Lucius Tarquinius Priscus, 616-579 BC, succeeded him as king. Well respected he was adopted by Ancus Marcius as his son, also appointing him guardian of his other sons. After the death of Ancus Marcius, Tarquinius Priscus was able to convince the People's Assembly that he should be elected king over Marcius' natural sons.