The Roman custom of liberally granting citizenship to "outsiders" was an odd practice when they first started doing it 2300 years ago and didn't become popular anywhere else until the United States was founded in the 1780s. The Romans made foreigners citizens largely because it was part of the myths surrounding the founding of Rome itself. The Romans believed that their original ancestors were outcasts from other tribes and cities and what made them unique, and strong, was the Roman ability to absorb large numbers of foreigners into the Roman state as equals. The Romans made it work, as did the Americans two thousand years later.