Complex sentence: Xanthippus was elected for the position of eponymous archon the following year (479/478 BCE), showing the esteem in which he was held at the time, as a large force of Persian infantry remained in Greece and Athens was still under threat.
Split into simpler sentences: Xanthippus was elected to the position of eponymous archon the following year (479/478 BCE), showing the esteem in which he was then held. At that time a large force of Persian infantry still remained in Greece and Athens was still under threat.