Iranian history spans thousands of years, beginning with ancient civilizations like the Elamites and the Persians. The Achaemenid Empire, founded by Cyrus the Great in the 6th century BCE, became one of the largest empires in history, known for its cultural achievements and administrative innovations. Over the centuries, Iran experienced various dynasties, including the Sassanids and later the Safavids, who established Shia Islam as the state religion. The 20th century saw significant changes, including the 1979 Islamic Revolution, which transformed Iran into an Islamic Republic, impacting its political and social landscape.