Themison was a tyrant

Tyrant

A tyrant was originally one who illegally seized and controlled a governmental power in a polis. Tyrants were a group of individuals who took over many Greek poleis during the uprising of the middle classes in the sixth and seventh centuries BC, ousting the aristocratic governments.Plato and...

 of Eretria

Eretria

Erétria was a polis in Ancient Greece, located on the western coast of the island of Euboea, south of Chalcis, facing the coast of Attica across the narrow Euboean Gulf. Eretria was an important Greek polis in the 6th/5th century BC. However, it lost its importance already in antiquity...

 who in 366 BC

366 BC

Year 366 BC was a year of the pre-Julian Roman calendar. At the time, it was known as the Year of the Consulship of Mamercinus and Lateranus...

 assisted the exiles of Oropus

Oropos

Oropos is a small town and a municipality in East Attica, Greece.-Geography:It is situated on the southern Euboean Gulf, opposite Eretria. Oropos is located N of Avlona and Athens, E of Thebes and SE of Chalcis. Oropos is linked with the road linking Nea Palatia and Sikamino...

 in recovering possession of their native city. They succeeded in occupying it by surprise, but the Athenians

Athens

Athens , is the capital and largest city of Greece. Athens dominates the Attica region and is one of the world's oldest cities, as its recorded history spans around 3,400 years. Classical Athens was a powerful city-state...

 having marched against them with their whole force, Themison was unable to cope with their power, and called in the Thebans

Thebes, Greece

Thebes is a city in Greece, situated to the north of the Cithaeron range, which divides Boeotia from Attica, and on the southern edge of the Boeotian plain. It played an important role in Greek myth, as the site of the stories of Cadmus, Oedipus, Dionysus and others...

to his assistance, who received possession of the city as a deposit, but afterwards refused to give it up.