An army of at least 250,000 troops, the last great Ottoman assault on Christian Europe, is sent by Sultan Mehmet IV from Edirne to Vienna, Austria. The city of Vienna has a great deal of strategic and symbolic importance and the Ottomans have been trying to capture it for quite some time.
For a couple of years prior to the formation of this army, Ottoman leaders have been providing financial and military assistance to minorities within the Hapsburg lands, particularly Hungarians and non-Catholics who are being persecuted as part of Leopold I's Counter-Reformation policies. Technically the Hapsburgs and the Ottomans are at peace, but the Ottomans are actively working to undermine Hapsburg power and stability - and almost certainly for the purpose of making conquest easier.
The Battle of Vienna wil take place in September after a two-month siege of the city. A combination of Hapsburg, Polish, and Lithuanian forces will defeat the Ottomans, leading to the ascendancy of the Hapsburg Dynasty in central Europe and the end of Ottoman incursions into Europe.
Key to the Hapsburg victory will be the timely arrival of a massive relief army of Poles under Jan III Sobieski, including over 20,000 cavalry. That entire compliment stages a single, massive charge downhill to break the Ottoman lines. It's the single largest cavalry charge in history and Jan III Sobieski rides at the head of one group of 3,000.