The Praetorian Guard were formed to serve and guard the Emperor in 27 BC, the year Augustus took the throne. He hand-picked them from the Roman Legions and they became the Praetorian Guard. Augustus knew that though they were the guard, they could have their own ideas about who should be Emperor. He allowed nine cohorts to form, each originally of 500 the 1,000 men in each. Despite measures taken by Augustus, they were successful in assasinating several Emperors, prominent among them was the Emperor Caligula in 41. The Guard was disbanded by the Emperor Constantine in the 4th century.