It is the Chi-Rho (a superimposition of the Greek letters χ and ρ), the symbol of Christ supposedly seen in the sky by the Emperor Constantine I before the Battle of the Milvian Bridge in AD 312. As the story goes, under this shining symbol, he saw the phrase ἐν τούτῳ νίκα, meaning "in this sign, conquer". After interpreting the vision, Constantine then instructed his soldiers, who were outnumbered, to paint the symbol on their shields and to press onwards to victory. The opposing Roman troops (commanded by the tetrarch Maxentius) were demoralized by this overtly Christian symbol and routed. With the victory at Milvian Bridge (which he attributed to God's sign), Constantine secured Rome and the ancient heartland of the Empire, and completed the restabilization of the Empire under his banner. The rest, including the foundation of the city of Constantinople, is history. Since then, the Chi-Rho has been an ever-enduring symbol of not only Christ and Constantine, but also of the Christianized Roman Empire itself.