Excerpts from Books and Wikipedia
"The first signs of the empires weakening became apparent during the Great Turkish War (1683-1699), which began with the second siege of Vienna and ended with the transfer of most of Hungary from Ottoman to Austrian hands." [Nothiger] "The Great Turkish War (...) or the War of the Holy League (…) was a series of conflicts between the Ottoman Empire and the Holy League consisting of the Habsburg Empire, Poland-Lithuania, Venice and Russia. Intensive fighting began in 1683 and ended with the signing of the Treaty of Karlowitz in 1699. The war was a defeat for the Ottoman Empire, which for the first time lost large amounts of territory. It lost lands in Hungary and Poland, as well as part of the western Balkans. The war was also significant in that it marked the first time Russia was involved in a western European alliance." [Wikipedia]