Thessalian League in the context of "Aeolic Greek"

Under the reign of Philip II (359–336 BC), the ancient kingdom of Macedonia, initially at the periphery of classical Greek affairs, came to dominate Ancient Greece in the span of less than 25 years, largely thanks to the character and policies of its king. In addition to utilising effective diplomacy and marriage alliances to achieve his political aims, Philip II was responsible for reforming the ancient Macedonian army into an effective fighting force. The Macedonian phalanx became the hallmark of the Macedonian army during his reign and the subsequent Hellenistic period. His army and engineers also made extensive use of siege engines. Chief among Philip's Thracian enemies was the ruler Kersebleptes, who may have coordinated a temporary alliance with Athens. In a series of campaigns stretching from 356 to 340 BC, Philip II managed to ultimately subjugate Kersebleptes as a tributary vassal, conquering much of Thrace in the process. Philip II also fought against the Illyrian king Bardylis, who threatened Macedonia proper, and against Grabos II and Pleuratus in Illyria (centred in modern-day Albania). In his newly conquered territories, he founded new cities such as Philippi, Philippopolis (modern Plovdiv, Bulgaria), Herakleia Sintike (Pirin Macedonia, Bulgaria), and Herakleia Lynkestis (modern Bitola, North Macedonia).

Philip II eventually campaigned against the city-state of Athens and her allies in the Aegean region, as well as Thebes after the decline of its hegemony in mainland Greece. In the defence of the Amphictyonic League of Delphi and in conjunction with the Thessalian League, Macedonia became a key player in the Third Sacred War (356–346 BC), defeating the Phocians, commanded by Onomarchus, at the Battle of Crocus Field in 352 BC. While poised to launch a direct assault on Athens in 346 BC, the Macedonian king was met with an Athenian embassy that arranged the Peace of Philocrates. As a result, Macedonia and Athens became allies, yet Athens was forced to relinquish its claims to the city of Amphipolis (in modern-day Central Macedonia).

Tagus (Ancient Greek: τᾱγός, τάγης) was a Thessalian title for a leader or general, especially the military leader of the Thessalian League. When occasion required, a chief magistrate was elected under the name of Tagus, whose commands were obeyed by all the four districts of Thessaly (Phthiotis, Thessaliotis, Histiaeotis, Pelasgiotis). He is sometimes called king ("basileus", Herodotus, V.63), and sometimes "archon" (Dionysius. V.74).

Accordingly, Pollux (I.128), in his list of military designations, classes together the Boeotarchs of the Thebans, the Kings of Sparta, the Polemarchs of the Athenians, (in reference to their original duties), and the Tagoi of the Thessalians. When Jason of Pherae was Tagus, he had an army of more than 8,000 cavalry and not less than 20,000 hoplites. When Thessaly was not united under a Tagus, the subject towns possessed more independence. Philip II of Macedon and his son Alexander the Great exercised control over Thessaly as elected Tagoi. In later times some states called their ordinary magistrates "Tagoi".