Phillip II of Macedon - First Foreign Leader of a Greek League
In the inaugural dissertation "Studies on War and Power Formation in Early Hellenism" (Justus Liebig University, Giessen) we find an important reference to the constant question of whether the ancient Macedonians were actually Greeks, as the modern Greek propaganda claims.
This construction that a foreign king was leader of a Greek league is an absolute novelty
In section "5.2.4" Personal union" we read the following passage about the father of Alexander the Great, Phillip II of Macedon:
... The area to which the Argead kings had been of special interest even before Philip was Thessaly, the bridging land between the Hellas of the great Poleis and Macedonia. The rule over Thessaly meant security to the south and was at the same time an offensive position, a gateway against Thebes or Athens, both of which had appeared as intervention powers in Macedonia. But Thessaly had more to offer than just its strategic location; Thessaly had a powerful cavalry, but also a high income.
When clashes broke out in Thessaly between the tyrants of Pherai and the Aleuads ruling in Larissa, both the Phocians under Onomarchus and Philip were asked to intervene. With that Philip ends up in the 3rd Holy War. As the elected general of the Thessalian federal contingent, he can finally defeat the Phocians on the crocus field (Diod. XVI, 35.4f .; 37.3; 38.1; Justin VIII, 2.1f.). Philip does not try to seize power over Thessaly by force, but allows himself to be elected Archon by the Thessalian Confederation (deliberately avoiding the discredited Tagos title). Later (probably) Philip completely restructured the Thessalian League after various difficulties by dividing the country into four tetrarchies; in return he guarantees the Thessalian cities their freedom, while the dynastic families lose their privileges (Demosth. 6.22; 8.62; 9.26). With short interruptions, Thessaly remained connected with Macedonia until the Battle of Kynoskephalai.
This construction that a foreign king was leader of a Greek league is an absolute novelty. ...
Source: Studies on war and power formation in early Hellenism, inaugural dissertation to obtain the doctoral degree in philosophy from Faculty 04 of the Justus Liebig University, Giessen presented by Frank-Gernot Schuffert, Bad Vilbel, page 175 - PDF Link (German)