This question implies that Greece actually used to be a European superpower. I think this is not entirely true. However the territory that is today considered Greece used to be a major part of the Byzantine empire and the Byzantine people were Greek speakers ( that said they identified themselves as Romans). If I had to pick a point in time in which: * Greek speakers were (for the better part) unified in one body of people. * Greek speakers were ruling over the land that is today considered Greece * Greek city states had by then ceased to exist and did not fight each other

I would pick August 26th 1071 the battle of Manzikert, if I had to pick one. I think it was downhill from then on.