For proving it we could remind about Egyptian inscriptions (the times of Ramses the Third). There might be mentioned some of Mycenaean Greece tribes. What did force them to leave Balkans? Second: Illiric tribes had sme troubles at that time and started moving to the South. I think there was some process in Central Europe - indeed that might be expansion of German (mix of German and Celtic) tribes. NB: some elements of Mycenaean culture and habits have been found even in Palestine. So we have signs of peoples movements - some people came to Greece and made a part of Mycenaean Greeks leave their homeland. And there was a community called itself Dorian (Laconia, Messenia, Crete etc.). Something was up...