Complex sentence: The Military Frontier was a province straddling the southern borderland of the Habsburg Monarchy and later the Austrian Empire and the Austro-Hungarian Empire, which acted as the "cordon sanitaire" against incursions from the Ottoman Empire.
Split into simpler sentences: The Military Frontier was a province straddling the southern borderland of the Habsburg Monarchy and later the Austrian Empire and the Austro-Hungarian Empire. It acted as the "cordon sanitaire" against incursions from the Ottoman Empire.