I will have to go with the Somme in 1916 and blame the defensive firepower of machine guns when properly employed.Or are you talking of the classical phalanx of antiquity?
Greek phalanxes were defeated by the Romans in 3 major battles : Pydna (168 BC), Cynoscephalae (197 BC) and Magnesia (190 BC).At Cynoscephalae and Magnesia, failure to defend the flanks of the Phalanx led to defeat; whilst at Pydna, the loss of cohesion of the Phalanx when pursuing retreating Roman soldiers allowed the Romans to penetrate the formation, where the latter's close combat skills proved decisive.
Of course the Roman Manipular formation is nothing more than a slightly modified Phalanx to begin with.
During the Republic. Agreed.The weaknesses of the Greek phalanx, which the Romans and others exploited, was the right flank (shields were held in left hand) plus the inability to turn (pivot) the formation quickly enough. As for who did it, I think this is arguable. The Persians were able to defeat the phalanx a few times.
Ok, so with the (ancient) phalanx, and answer I heard was that its demise came in Cynoscephalae when Romans were able to penetrate it by attacking its flank (because the phalanx's inherent weakness is inability to efficiently turn) and also by attacking it on uneven ground, where the phalanx was unable to maintain its dominance. The other battles at Pydna and Magnesia that Omer mentions might have led to similar situations, but I'm guessing the decline of the formation began in Cynoscephalae.
But the Phalangeal style of formation persists to this day. What do you think an Infantry squad on line is except for a greatly modified Phalanx. It persists because of its effectiveness.
An Athenian phalanx was defeated by a different type of army during the Peloponnessian War. In 429 B.C., the Olynthians, with calvalry and lightly armed troops, beat an Athenian hoplite force.
The key to defeating the Greek phalanx was breaking just one guy in the front line and gaining access to the interior of the formation. Once inside it was child's play to totally destroy the formation because the Hoplites were so crowded within the formation that they could only go forward and had no room to defend themselves.
I'd say the key was cavalry or any other non-close (is that a word?) combat…like peltists or javelin throwers. The hoplite was virtually undefensible against that. Once cavalry broke a wing, that was pretty much it. But you do make a good point for hoplite-to-hoplite fighting.What I thought was cool, is pushing back the center. It looked like the one doing the pushing was winning, but they just ended up getting surrounded with no means of escape.
Neither cavalry nor ranged/light infantry could break a phalanx. Both could nip at it and wear it down but the point of phalangeal combat was to make battle quick and decisive, neither cavalry or light infantry could be decisive when fighting Hoplites formed in battle array. Except for a short interregnum during the middle ages heavy infantry as been THE decisive combat arm for almost all of recorded history. Cavalry has never been able to break an infantry formation. Anyone with riding experience can tell you that horses will not run at or into an immovable object such as that presented by formed infantry, they have to be tricked into doing it.
The obvious response to an attack by a phalanx was a first strike by light armed spearmen and archers. Their missiles would break the ranks of the attacking phalanx. At the same time, cavalry could be placed on the wings, which could attack the enemy's rear once the battle had started.
Athens also lost Syracuse because the Syracusans had a formidable cavalry (that's not the only reason they lost, but it was a major factor, IMO) Thucydides 7.85-87