Greece's mountainous terrain favored a defensive strategy, while the high ranges of the Rhodope, Epirus, Pindus and Olympus mountains offered many defensive opportunities. Air power was required to protect defending ground forces from entrapment in the many defiles. Although an invading force from Albania could be stopped by a relatively small number of troops positioned in the high Pindus mountains, the northeastern part of the country was difficult to defend against an attack from the north.