The war between Athens and Macedon erupted in 356, when Phillip moved to strike a series of Athenian ports that had refused goods to the Macedonian Kingdom. Seeing a opportunity to seize the massive gold mines and deposits in the North, Phillip moved his army against the ports, which were relatively undefended. Over the next four years, Macedon successfully defeated the Athenian allies and satellite cities, eventually launching attacks on Athenian colonies, and defeating a series of counter-invasions led by the navy.
Phillip was able to muster a massive force, nearly twice the size of his initial force, and marched on Pagasae, a Athenian base of operations. After defeating a large garrison force, he was able to destroy any naval support that Athens could give to its land army, under Helmetrus. Phillip split the allied armies, and was able to single out the Athenian army, which, after a series of small losses, numbered much less then Phillips army.
As the battle was to be engaged along the coast, and Phillip was very much reliant on his cavalry, Helmetrus heavily supported his right flank (the one's farthest away from the coast.) He also moved his mercenary troops to a defensive position away from the army, in order to prevent the Macedonian cavalry from swinging around. The Macedonian King noticed the maneuver, and challenged Helmetrus with his own. Phillip moved all of his light cavalry to his left side (nearest to the water), as they would be much more effective in a direct cavalry charge, especially with Helmetrus's weakened left. The Macedonian heavy cavalry was moved to Helmetrus's strong side, as the Athenians lacked decent cavalry support on their strong right.
As Philip advanced his hoplite line, he moved his light cavalry forward, to engage the Athenian right flank. Helmetrus rushed his archers to that side, and as the horses slowly advanced through the shallow water, they were caught in heavy arrow fire. By the time the light cavalry had reached the hoplites, nearly half of them had been drowned or killed in the fire.
Despite the defeat of the left side, the Hoplites continued to advance, leaving a undefended gap near the coast. Helmetrus exploited this move, and marched his weaker side up, enveloping the coastal side of the Macedonian army. Phillip pulled his heavy cavalry around to support the weak side, leaving the Macedonian right open to attack.
With Helmetrus's forces stronger on his left side, he moved his army forward, offering battle to all except his center. Helmetrus allowed his forces to be spread thin, and eventually fall back. However, he forced his flanks to dig in a few hundred feet from the center, while the Macedonian's continued to advance head strong. The result, was the sudden envelope of the Macedonian center, which, under pressure from three sides, was massacred. As the Macedonians poured in, Helmetrus pulled his reserves and cavalry forward, and sandwiched in the Macedonians, completely destroying the organization of Phillips army.
In a final motion, Helmetrus transformed his envelope into a circle, and entrapped nearly 23,000 Macedonians. Nearly 16,000 of them died in the following entrapment, effectively destroying Phillips army.
Surrender and Aftermath
Seeing the immense slaughter of the battlefield, Phillip ordered the remains of his army to lay down their weapons in surrender. Some patches of Macedonians refused to end fighting, but the majority of the force complied with their King's demands.
Helmetrus and Phillip met in camp the following evening, as the dead were being carried out. Overcome with grief, Phillip accepted Athenian peace terms, and surrendered 1/10th of his territory, 1/3 of his treasury, and his young son, Alexander as a hostage.
Helmetrus returned home in triumph, and his political powers were furthered extended, with his ability to now freely sack and hire new officials.