As the 16th century progressed, Ottoman naval superiority was challenged by the growing sea powers of western Europe, particularly Portugal, in the Persian Gulf, Indian Ocean and the Spice Islands. With the Ottoman Turks blockading sea-lanes to the East and South, the European powers were driven to find another way to the ancient silk and spice routes, now under Ottoman control. On land, the Empire was preoccupied by military campaigns in Austria and Persia, two widely separated theatres of war. The strain of these conflicts on the Empire's resources, and the logistics of maintaining lines of supply and communication across such vast distances, ultimately rendered its sea efforts unsustainable and unsuccessful. The overriding military need for defence on the western and eastern frontiers of the Empire eventually made effective long-term engagement on a global scale impossible.