The term "tetrarchy" (from the Greek τετραρχία "leadership of four [people]") describes any form of government where power is divided among four individuals, but in modern usage usually refers to the system instituted by Roman Emperor Diocletian in 293, marking the end of the Crisis of the Third Century and the recovery of the Roman Empire. This Tetrarchy lasted until c. 313, when internecine conflict eliminated most of the claimants to power, leaving Constantine in the East and Licinius in the West.
Although the term "tetrarch" was current in antiquity, it was never used of the imperial college under Diocletian. Instead, the term was used to describe independent portions of a kingdom that were ruled under separate leaders. The tetrarchy of Judaea, established after the death of Herod the Great, is the most famous example of the antique tetrarchy. The term was understood in the Latin world as well, where Pliny the Elder glossed it as follows: "each is the equivalent of a kingdom, and also part of one" (regnorum instar singulae et in regna contribuuntur).
Thessaly or Thessalia (Greek: Θεσσαλία, Thessalian: Πετθαλία, Petthalia) was one of the traditional regions of Ancient Greece. During the Mycenaean period, Thessaly was known as Aeolia, a name that continued to be used for one of the major tribes of Greece, the Aeolians, and their dialect of Greek, Aeolic.
At its greatest extent, ancient Thessaly was a wide area stretching from Mount Olympos to the north to the Spercheios Valley to the south. Thessaly is a geographically diverse region consisting of broad central plains surrounded by mountains. The plains are bounded by the Pindos Mountains to the west, Mount Othrys to the south, the Pelion and Ossa ranges to the east, and Mount Olympos to the North. The central plains consist of two basins, the Larisa basin and the Karditsa basin, drained by the Pineios River into the Vale of Tempe. The Pagasetic Gulf in southeastern Thessaly was and is the only body of water suitable for harbours in region.
Strictly speaking, Thessaly refers primarily to the central plains inhabited by the Thessalians in antiquity. The plains were divided in antiquity into four administrative regions called tetrads: Pelasgiotis, Phthiotis, Thessaliotis, and Hestiaiotis. In its broader sense, Thessaly also included the surrounding regions called the perioikoi, which were regions inhabited by different ethnic groups that were closely tied to the Thessalians either as subordinates, dependents, or allies. The perioikoi were composed of Perrhaibia, Magnesia, Achaea Phthiotis, Dolopia, Ainis, Malis, and Oitaia. The three largest cities in Thessaly were Larisa (Pelasgiotis), Pherai (Pelasgiotis), and Pharsalos (Phthiotis).
The Herodian Tetrarchy was formed following the death of Herod the Great in 4 BCE, when his kingdom was divided between his sons as an inheritance. Judea, the major section of the tetrarchy, was transformed by Rome in 6 CE, abolishing the rule of Herod Archelaus, and forming the Province of Judea by joining together Judea proper (biblical Judah), Samaria and Idumea (biblical Edom). However, other parts of the Herodian Tetrarchy continued to function under Herodians. Thus, Philip the Tetrarch ruled Batanea, with Trachonitis, as well as Auranitis until 34 CE (his domain later being incorporated into the Province of Syria), while Herod Antipas ruled Galilee and Perea until 34 CE.
At the time of his death, Herod ruled over most of the South Western Levant, as a client-state of the Roman Empire. Antipas was not Herod's first choice of heir. That honor fell to Aristobulus and Alexander, Herod's sons by the Hasmonean princess Mariamne. It was only after they were executed (c. 7 BCE), and Herod's oldest son Antipater was convicted of trying to poison his father (5 BCE), that the now elderly Herod fell back on his youngest son Antipas, revising his will to make him heir. During his fatal illness in 4 BCE, Herod had yet another change of heart about the succession. According to the final version of his will, Antipas' elder brother Archelaus was now to become king of Judea, Idumea and Samaria, while Antipas would rule Galilee and Perea with the lesser title of tetrarch. Philip was to receive Gaulanitis (the Golan Heights), Batanaea (southern Syria), Trachonitis and Auranitis (Hauran).