In the second half of the 4th century BC, Armenia was divided by the Achaemenids into Greater and Lesser Armenia. From the beginning of the 4th century BC, the weakened power of the Achaemenids embarked upon the path of decay.
In 334 BC, Macedonian King Alexander the Great crossed the straits of the Black Sea, invaded the possessions of the Persian kings, and within one year seized the western possessions of the Achaemenid state.
In 331 BC, in the Battle of Gaugamela, Alexander the Great defeated the Persian army led by the king of Persia Darius III. Soon, Darius III was killed by his own entourage.
In the battle of Gaugamela, Armenian troops from Greater and Lesser Armenia fought on the Persian side. A squad from Greater Armenia was led by satrap Yervand.
Having conquered the Achaemenid state and made a campaign to the East towards Central Asia and India, Alexander the Great created a gigantic state, choosing Babylon as its capital.
As a result of the conquests of Alexander the Great in Western Asia, the Hellenistic period began, the most important feature of which was the widespread and fruitful interaction of the Hellenic (Greek) civilization with the local Eastern civilizations.
The troops of Alexander the Great did not invade Armenia. A participant of the Battle of Gaugamela, Yervand, reigning in Armenia, restored the statehood whose traditions had been laid in the 6th century BC.
At this time, Lesser Armenia also sought independence. The Armenian ruling dynasty of Yervandunis, which was related to the Achaemenids, has accumulated considerable experience of state governance of Armenia.
This significantly strengthened the position of the Armenian Kingdom in the system of the Hellenistic states that formed after the collapse of Alexander the Great’s empire. After the sudden death of Alexander of Macedon in 323 BC, his empire was divided between his generals.
Source: History of the Armenian people in questions and answers (Russian-Armenian (Slavic) University)