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The Imperial guard was a formation of the army, and Emperors often deployed them in the field. The Praetorian cohorts often accompanied Emperors on the campaign, where they protected their sovereign. For example, they accompanied Trajan on his campaign in Dacia, and this is memorialized on the Arch of Trajan in Rome. It was not uncommon for the Imperial Bodyguard to take part in the battle and they were regarded as an elite unit, who was often used when the outcome of a battle was uncertain. On one occasion they accompanied Germanicus as he quelled a great mutiny among the legions on the Rhine, who were threatening the new Emperor Tiberius. They helped Germanicus to persuade the legionnaires to return to their camps and restore discipline .<ref> Tacitus, x, v </ref>. For many Emperors, they were seen as the last line of defense against a mutinous army or the legions of a rival.