Why did the Persian Empire feel so threatened by the expansion of ancient Grecce?

Greece was in fact the Word used for over 2,000 independent city-states. Persia expanded its empire to take in the Greek cities in Asia Minor. Then when the cities in mainland Greece began to interfere on the side of their daughter-cities in Asia Minor, Persia attempted to bring those cities within its empire to keep them quiet. It failed in this after 50 years of war, gave up and turned back to ruling its original empire in Asia. The disruption from the Greek cities and their continuing warfare between each other. This led Persia to impose the King's Peace, threatening retribution to any further disruptions. This ended when Macedonia came to dominate Greece, and then used these resources to invade and takeover Persia under the leadership of Alexander the great, king of Macedonia.