On Oct. 28, 312—17 centuries and a couple months ago—the armies of two
rival Roman emperors clashed at the Battle of the Milvian Bridge just
outside Rome. One of the leaders was Constantine, and before the battle
he ordered his soldiers to paint a symbol made up of the Greek
characters chi and rho on their shields, assuring them
that it was a sign of divine support. He and his men won, propelling
Constantine toward taking control of the entire empire. To this day
their victory is commemorated on the Arch of Constantine near the
Colosseum.
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WSJ