By contrast, the Habsburg frontier had settled into a reasonably permanent border, marked only by relatively minor battles concentrating on the possession of individual fortresses. The stalemate was caused by a stiffening of the Habsburg defences and reflected simple geographical limits: in the pre-mechanized age, Vienna marked the furthest point that an Ottoman army could march from Istanbul during the early spring to late autumn campaigning season. It also reflected the difficulties imposed on the Empire by the need to support two separate fronts: one against the Austrians (see: Ottoman wars in Europe), and the other against a rival Islamic state, the Safavids of Persia (see: Ottoman wars in Near East).