Invested a large amount of resources in invading Britain but lost them to the pictish forces in Scotland. This failed attempt marked the beginning of stagnation for the Romans in terms of land exspansion, like wise it also signaled economic termal and political strife as everything in Rome was based around a constant supply of I'll gotten gains from their exspansions. Now with Rome unstable, a large portion of the barbarians that sorounded Rome started to invade. Even worse the Romans resorted to hiring the Francs (may be spelt wrong) and the Francs took advantage of this, grabbing land of there own to form what is now France. You see the fall of Rome wad not just one event, it was the result of a series of events. But I guess you could sumarise as the Roman war machine running out of steam.