Before Alexander the Great ruled, King Phillip the II of Macedonia (382-336 BC) developed the Macedonian military and wanted to integrate all the Greek people in the Peloponnesian Peninsula and Asian Minor. In 338 BC, Phillip defeated all the city-states except for Sparta and the city-states sent men to him to help conquer the Persians. 

Alexander was to be the next king of Macedon and was tutored by Aristotle. His goal was to unite all of the civilized world under one Greek ruler. Alexander was not known as a brutal conqueror, and many kingdoms allowed him to take over and soldiers surrendered to his armies willfully. 

In 334 BC, Alexander conquered Asia Minor, and the next year he continued south, taking over Syria, Phoenicia, Palestine, and Egypt. After the Siege of Tyre in 332 BC, he invaded the Mesopotamian region and officially defeated the Persian Empire in 331. Alexander became the successor to the Persian Kingdom. 

As he pushed eastward, Alexander established cities and made it as far as modern-day India. Many people thought of Alexander as a god and on his coins there were often lightning bolts. In 323 BC, Alexander the Great died, leaving his conquests to be fought over for four decades after his death. The successor kingdoms of his empire are known as the Hellenistic Kingdoms, and his former generals were the new kings. The most prominent of the new areas were the Kingdom of Greece and Macedon, the Seleucid Kingdom, and the Kingdom of Egypt. 

This time period is known as the Hellenistic Period. During this period, scientists started studying medicine to try to get a better understanding of the human body. Hippocrates (460-370 BC) had at least 60 texts about medicine. He started Greek medical practices and many later scientists followed in his footsteps. Herophilus of Chalcedon continued these studies and even used criminals and live subjects. Notably, he was one of the first scientists to study the human nervous system.