One of the bigger mistakes in military history. Never underestimate "barbarians". 

Vietname, the era of guerilla warefare and helicopters!

You could put up an argument that the roman empire lasted so long simply because there wernt as many threats as other great civilisations to follow it..And they were so advanced in tactics and weaponry...Infact caesar said then when he reached the english soil , 'the natives lived in such miserable squaler that there wasnt a point in invadin' (that wasnt a direct quote).  If you think about it the romans had the persians as an enemy but apart from that not much else.  But form then on more and more nations developed, like gaul and the mithridates..

thats my theory on why the lasted so long,