The Germanic invasions began late in the 2nd Century. As you recall, Marcus Aurelius died on the frontier, and was succeeded by his dissolute son, Commodus. The 3rd Century was bad for Rome, and it took the stern leadership of Diocletian is reestablish the authority of the state.
it took the stern leadership of Diocletian is reestablish the authority of the state.
And by splitting the empire in two he sealed the western empires fate.