In Greco-Roman antiquity, the ruler of a principality, originally the ruler of one-quarter of a region or province. The first tetrarchs ruled the four tetrarchies of Thessaly under Philip II of Macedonia. Tetrarchs ruled in Galatia (in Asia Minor) before the Roman conquest (169 BC) and still later in Hellenized Syria and Palestine, where the h1 denoted the semi-independent ruler of a divided kingdom or minor district. Herod the Great's realm after his death (4 BC) was ruled by his three sons, two of whom were called tetrarchs.
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It can also refer, more loosely, to