What we learned ... from the battle of Thessaly.

In 353 BC Philip of Macedon marched into Thessaly at the request of the Thessalian League to attack the city of Pherae, which had allied itself with Phocis against Thebes in the Third Sacred War. Upon learning of Philip's intervention, Lycophron, the ruler of Pherae, asked Phocis for military help. Onomarchus, commander of the Phocian army, was besieging Chaeronea when the request reached him. He immediately dispatched his brother with 7,000 troops. But Philip intercepted the Phocian relief force and destroyed it, opening the road south to Phocis. Getting word of Philip's victory, Onomarchus broke off his siege and marched north to head off the Macedonians.

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The Phocian phalanx was a traditional Greek infantry composed of citizen hoplites and mercenaries equipped with swords, short spears, body armor and large-diameter (38-inch) shields. Philip's Macedonian phalanx comprised trained professionals armed with the 15-foot pike (sarissa), which required two hands to wield. His phalangites wore helmets and leg greaves but no body armor, and their small-diameter (24-inch) shields hung from a shoulder strap. In fight phalanx formation, they were virtually impregnable. But in close combat, the unwieldy pike, small shield and lack of armor put the Macedonian infantryman at great disadvantage against the Phocian hoplite.

Diodorus, in The Reign of Philip II, wrote that the two armies initially skirmished, with the Macedonians getting the worst of it. Philip continued his advance, however, so Onomarchus switched to the defensive and set a trap. "In deploying against the Macedonians with a crescent-shaped mountain at his back," Polyaenus wrote in Stratagems of War, "Onomarchus concealed on the summits on each side infantry and stone-throwing artillery and advanced his force into the plain lying beneath." As Philip deployed his troops for battle, he discovered that his infantry spanned the plain between the summits, leaving no room to deploy flanking cavalry. Onomarchus' tactical deployment had taken it out of the fight, forcing Philip into a pure infantry battle.

Philip sent his pike phalanx to the attack. "The Phocians pretended to flee into the central area of the mountains," Polyaenus wrote. The Macedonians closed to pin their fleeing enemy against the hills, and the Phocians drew them deep into a cul-de-sac. Then Onomarchus' stone-throwing catapults opened fire on Philip's packed phalanx.

The Macedonians dropped their pikes and raised their small shields for protection. "At that very moment," Polyaenus wrote, "Onomarchus signaled for the Phocians to turn and engage the enemy, while some [catapults] were shooting from behind, others from the front.... The Macedonians, being crushed by stones, retreated."

Philip of Macedon had suffered his first defeat, at the hands of an unknown Phocian general. But Philip was not a man to easily forgive slights, and a year later he returned to Thessaly and defeated Onomarchus at the Battle of Crocus Field. Philip went on to conquer Greece--and never lost another battle.

Lessons:

* Use the terrain. By narrowing Philip's tactical box, Onomarchus took his cavalry out of the fight, depriving the Macedonian army of half its combat power.

* Belie your strength and disposition. Onomarchus concealed his catapults and reserves in the hills and revealed only part of his force on the open plain.

* Be vigilant. Philip failed to detect the ambush that almost destroyed his army. As Herodotus once wrote, "The assassins hide even in the clouds."

* Improvise. Onomarchus' use of siege catapults against Philip's phalanxes was a stroke of genius. In so doing, he invented field artillery as a combat branch.

* Change the tactical dynamics. The Macedonian pike phalanx was impregnable as long as it remained intact. By shattering it with artillery, Onomarchus pressed his advantage in close combat.

* Follow through. Onomarchus failed to send his cavalry in pursuit, allowing Philip to withdraw and rebuild. A year later he faced Philip again--and lost.

* Think long-range. Onomarchus didn't realize Philip's incursion was part of broader invasion plans. Philip didn't consider the impact defeat at Thessaly would have on his alliances, which unraveled.

* Check your ego. In six years of campaigning, Philip had won every battle he fought. Thus, he failed to respect Onomarchus' ability even though Phocian forces had won a prior skirmish.