At the same time that Athens was under attack from Sparta, the city-state also suffered a terrible plague, beginning in 430 B.C. The plague killed about a third of Athens’s citizens. This contributed to Athens’s eventual defeat at the hands of its Spartan rivals during the Peloponnesian War: After the plague had claimed so many lives, Athens simply couldn’t muster the military leadership and strength that it needed to defeat the increasingly powerful Peloponnesian League. At the end of the fighting, in 404 B.C., Athens was in ruins. Sparta became the dominant city-state in the Greek world. But conflicts continued among the city-states, and Thebes eventually defeated Sparta in 371 B.C. As a result of the ongoing warfare, the Greek economic conditions declined, and the gap between rich and poor widened. The public spirit that had been the hallmark of the golden age of Greece disappeared. In short, the city-states were no longer the glorious entities they had once been.

While the Greek city-states were in decline, Greece’s neighbor to the north, Macedonia, was growing more powerful. In 353 B.C. Macedonian king Philip II (382–336 B.C.) launched an attack on Greece. The war that resulted did not end until 338 B.C. when Greece was finally conquered. The Macedonian victory was only the first part of Philip’s plans; he believed a combined Macedonian-Greek army could defeat the powerful Persians. But he was not to see this happen. When Philip was killed by a Macedonian in 336 B.C., he was succeeded by his 20-year-old son, Alexander (356–323 B.C.), a man history would come to call “the Great.” Alexander carried out his father’s plan to invade Persia. And two years after Philip’s death, Alexander began a 10-year campaign that ultimately conquered the Persian Empire. By the time he was 30 years old, Alexander had conquered much of the known world, expanding his empire from Egypt to India.