How did Cyrus manage to expand the Persian holdings so dramatically during his lifetime?

Answer 1

As a military leader, Cyrus left an everlasting legacy on the art of leadership and decision-making and he attributed his success to "Diversity in counsel, unity in command." Cyrus the Great respected the customs and religions of the lands he conquered. It is said that, in universal history the role of the Achaemenid empirefounded by Cyrus lies in its very successful model for centralized administration and establishing a government working to the advantage and profit of its subjects. In fact, the administration of the empire through satraps and the vital principle of forming a government at Pasargadae were the work of Cyrus. Aside from his own nation, Iran, Cyrus also left a lasting legacy on Jewish religion (through his Edict of Restoration), human rights, politics, and military strategy, as well as on both Eastern and Western civilizations.

Answer 2

He allied himself with the Medes and used the combined power to takeover Weatern Asia from the Babylonian and Assyrian Empires.