 In the 3rd century, the Roman frontiers weakened against the Germanic tribes across the Rhine and Danube, and the Sassanid Empire across the Euphrates increased its own attacks.When the Persians under Shapur I invaded Mesopotamia, the young emperor opened the doors of the Temple of Janus for the last time in Roman history, and sent a large army to the East.The Sassanids were driven back over the Euphrates and defeated in the Battle of Resaena (243).