Quran has informed about the future in a miraculous way after the defeat of the Romans. Usually the defeated nations can not stand up strongly in short period to defeat those who defeated them. Allah said in Sura Rum: And after being defeated they (Romans) will defeat (their opponents) within few years Bidh'i بضع سنين which is between three to nine or ten years. The exact number of years was not mentioned because the victory of the Romans on their enemies took few years through many battles and was not in just one year.
'The battles took few years till the Rum defeated their enemies. Quran said: في بضع سنين Within or during few years, which is exactly what happened.