Beginning in the 1530s, the size of the Janissary corps began to dramatically expand, a result of the rapid conquests the Ottomans were carrying out during those years. Janissaries were used extensively to garrison fortresses and for siege warfare, which was becoming increasingly important for the Ottoman military. The pace of expansion increased after the 1570s, due to the initiation of a series of wars with the Safavid Empire and, after 1593, with the Habsburg monarchy. By 1609, the size of the corps had stabilized at approximately 40,000 men, but increased again later in the century, during the period of the Cretan War (1645–69) and particularly the War of the Holy League (1683–99).