1529 AD
In 1529, Sultan Suleiman the Magnificent led a massive Ottoman army of approximately 150,000 men on a campaign that swept through Hungary (capturing Buda in 1526) and reached the gates of Vienna, the capital of the Holy Roman Emperor Charles V. The Ottomans besieged Vienna for two weeks but were forced to retreat due to supply problems, harsh weather, and the approach of winter.
The failure of the first siege did not end the Ottoman threat. The Turks would control Hungary for nearly 150 years and would march on Vienna again in 1683. The Ottoman Empire at its height controlled the Balkans, the Middle East, North Africa, and parts of Eastern Europe, making it the most powerful military force in the world.
The Catholic Church viewed the Ottoman threat as an existential danger to Christendom. Pope Clement VII called for a crusade, but the Reformation had fractured Europe and made unified action impossible. Key Sources: Busbequius, Turkish Letters; Roxelana's correspondence; Ottoman military records