Battle of Chaeronea (338 BC) - Wikipedia

On August 2, 338 BC, the Macedonian army led by Philip II defeated the combined forces of Athens and Thebes in the Battle of Chaeronea, securing Macedonian hegemony in Greece and the Aegean. From the article:

"Philip had brought peace to a war-torn Greece in 346 BC, by ending the Third Sacred War, and concluding his ten-year conflict with Athens for supremacy in the north Aegean, by making a separate peace. Philip's much expanded kingdom, powerful army and plentiful resources now made him the de facto leader of Greece. To many of the fiercely independent Greek city-states, Philip's power after 346 BC was perceived as a threat to their liberty, especially in Athens, where the politician Demosthenes led efforts to break away from Philip's influence. In 340 BC Demosthenes convinced the Athenian assembly to sanction action against Philip's territories and to ally with Byzantium, which Philip was besieging. These actions were against the terms of their treaty oaths and amounted to a declaration of war. In summer 339 BC, Philip therefore led his army towards South Greece, prompting the formation of an alliance of a few southern Greek states opposed to him, led by Athens and Thebes.

After several months of stalemate, Philip finally advanced into Boeotia in an attempt to march on Thebes and Athens. Opposing him, and blocking the road near Chaeronea, was the allied Greek army, similar in size and occupying a strong position. Details of the ensuing battle are scarce, but after a long fight the Macedonians crushed both flanks of the allied line, which then dissolved into a rout.

The battle has been described as one of the most decisive of the ancient world. The forces of Athens and Thebes were destroyed, and continued resistance was impossible; the war therefore came to an abrupt end. Philip was able to impose a settlement upon Greece, which all states accepted, with the exception of Sparta. The League of Corinth, formed as a result, made all participants allies of Macedon and each other, with Philip as the guarantor of the peace. In turn, Philip was voted as strategos (general) for a pan-Hellenic war against the Persian Empire, which he had long planned. However, before he was able to take charge of the campaign, Philip was assassinated, and the kingdom of Macedon and responsibility for the war with Persia passed instead to his son Alexander."