The Roman Empire lasted for nearly fifteen hundred years between 709 and 2206 AUC, but things could have been so very different. There are many points at which, had battles gone other ways, had different Emperors been chosen, the Empire could have fallen.
The setbacks at the hands of Arabs and Mongols are well known as is the final fall to the Turks, but few of us remember that the Empire could so easily have fallen as early as 762 AUC. At that time, Germania Magna had barely been in Roman hands for a generation while Britannia, Hibernia, Tulia and Scandia had not even been added to the Empire, let alone the later conquests of Samogitia, Scythia and Colchis.
The Government of Germania Magna was in the hands of Publius Quinctilius Varus, an able general, as it was to turn out, but one of the worst governors in the history of the Empire, having to put down revolts in Africa, in Syria, and after Germania Magna, in Judaea. Those other revolts never threatened the power of Rome, that in Germania Magna did.
The German leader, Arminius ambushed Varus' legions (and of course a civilian train travelling with Varus) in the Teutoberg Forest and on the 9 September 762, routed the Legions and put civilians to the sword. It is this very viciousness that was Arminius' undoing. The confusion gave Varus time to regroup and gave his troops the fire in their belly needed to avenge the deaths of their children and their women. Had Arminius carried on in pursuit of the shattered Legions, he had the strength of numbers to overcome what was, until Varus rallied the troops, a scared and disorganised rabble.
With Varus' legions dead, it seems likely that Germania Magna itself would fall, pushing the frontier back to the Rhine. Now obviously the Rhine is more easily defensible than the Elbe, Oder, or Vistula, the result of the Loss of Germany was, there would be no need to take Britannia or Scandia to supply the German cities via rivers that empty into the Mare Balticum. One problem the Empire often had was an inability to see threats before they arrived. How long would it be before Rome was beset by wave after wave of invasion, not only from Germans and Slavs, but also from Britannia and Scandia, realms then beneath notice.
Arminius' failure to finish off Varus' legions on September 9 not only cost him a German Empire, but might well have saved the Roman Empire from destruction long before it came in our world. And without the Empire, could civilisation have survived the coming of the Mongols? I doubt it. Varus saved us all.