The city maintained independence as a city-state until it was annexed by Darius I in 512 BC into the Persian Empire, who saw the site as the optimal location to construct a pontoon bridge crossing into Europe as Byzantium was situated at the narrowest point in the Bosphorus strait. Persian rule lasted until 478 BC when as part of the Greek counterattack to the Second Persian invasion of Greece, a Greek army led by the Spartan general Pausanias captured the city which remained an independent, yet subordinate, city under the Athenians, and later to the Spartans after 411 BC. A farsighted treaty with the emergent power of Rome in which stipulated tribute in exchange for independent status allowed it to enter Roman rule unscathed. This treaty would pay dividends retrospectively as Byzantium would maintain this independent status, and prosper under peace and stability in the Pax Romana, for nearly three centuries until the late 2nd century AD.