The battle of Mantinea of 207 BC was the most significant battle of the First Macedonian War, although it involved none of the main participants in that war. The war had broken out after Philip V of Macedonia decided to ally himself with the victorious Hannibal, then at the height of this fame after the battle of Cannae. Most of the fighting on land was between the forces of Philip and those of the Aetolian League, Rome’s ally in Greece, but fighting also took place between Philip’s ally the Achaean League and Rome’s allies in Sparta (part of the longer Spartan-Achaean Wars).
In the early years of the war the Achaean army had performed poorly, forcing Philip V to come to their aid in several occasions. This began to change when Philopoemen of Megalopolis, an experienced soldier who had spent ten years serving as a mercenary commander on Crete. On his return to Achaea he was appointed Hipparch, or cavalry commander. After reforming the cavalry (210-9) he was appointed General (208-7), and reformed the infantry, apparently introducing the latest Macedonian tactics and equipment.
The reformed army was soon put to the test. After capturing Tegea Machanidas of Sparta began to advance towards Mantinea. The Achaeans took up a position on the narrow plain outside the city, where so many battles had been fought. Philopoemen is said to have positioned his troops behind a wide ditch. The two armies were probably of about the same time – the Spartans with 15,000 men and the Achaeans with 15-20,000.
The battle began with a rare example of the use of siege engines as field artillery. Machanidas led his mercenaries in an attack on the Achaean left wing. This was made of some of Philopoemen’s mercenaries and light troops. They broke, and were pursued back towards Mantinea.
Despite this initial setback Philipoemen managed to reform his line before the Spartan phalanx attacked. As the Romans would soon discover, the phalanx was a powerful force but was very vulnerable if disrupted. This would appear to be what now happened, with the Achaean phalanx attacking the Spartans as they were crossing the ditch and defeating them.
At some point during this phase of the battle Machanidas returned to the battlefield with his mercenary troops. In the fighting that followed Machanidas was killed, possibly by Philipoemen himself.
The Spartans were said to have lost 4,000 men during the battle. Philopoemen was able to recapture Tegea, and then invade Laconia. His success also allowed Philip to focus his efforts on the Aetolian League, at a time when Roman interest in the war had waned. In the following year Philip and the Aetolians made peace. With no allies in Greece, and their main war aim achieved (preventing Philip from leaving Greece), in 205 the Romans also made peace with Philip (Peace of Phoenice). This would be no more than a temporary truce. The Romans never forgave Philip for attacking them when they were at their most vulnerable, and only five years later the Second Macedonian War broke out.