In 340 BC, Philip assembled a large Macedonian army and invaded Thrace. He left 16 year old Alexander with the power to rule Macedonia in his absence as regent. But as the Macedonian army advanced deep into Thrace, the Thracian tribe of Maedi bordering north-eastern Macedonia rebelled and posed a danger to the country. Alexander assembled an army, led it against the rebels, and with swift action defeated the Maedi, captured their stronghold, and renamed it Alexandroupolis.