Parthia was an Iranian civilization situated in the northeastern part of modern Iran. Their power was based on a combination of the guerrilla warfare of a mounted nomadic tribe, with organizational skills to build and administer a vast empire – even though it never matched in power and extent the Persian empires that preceded and followed it. The Parthian Empire was led by the Arsacid dynasty, which reunited and ruled over significant portions of the Near East and beyond, after defeating and disposing the Hellenistic Seleucid Empire, beginning in the late 3rd century BC. It was the third native dynasty of ancient Iran (after the Median and the Achaemenid dynasties). Parthia had many wars with the Roman Republic (and subsequently the Roman Empire), which marked the start of what would be over 700 years of frequent Roman-Persian Wars.