For close to a thousand years - roughly from 700 BC to AD 300 - most of Europe
was cooler and moister than it is now.
The moister climates shifted the boundaries between grassland and forest in eastern Europe. The grassy steppes in southern Russia and Hungary became wooded country.
Nomadic tribes had long roamed the grasslands in the north from where they periodically descended on the civilized lands to the south. Which of these lands they invaded at any given time would depend on where they happened to be located - and this in turn, was influenced by the boundary between steppe, where the nomads thrived, and forests, where their herds could find little pasturage.
The classical age of Greece coincided with the moister period with woodland covering the Balkans, forcing the nomads to find pastures further to the east.
The eastwards - shifting nomads ceased to threaten Greece, they did, however, attack Persia to which they were a constant threat - serious enough perhaps, to tip the military balance between Persian and Greeks.