Task: Question Generation
Input (English): Although the Palaeologi emperors recaptured Constantinople from the Western Europeans in 1261, they were never able to regain control of much of the former imperial lands. They usually controlled only a small section of the Balkan Peninsula near Constantinople, the city itself, and some coastal lands on the Black Sea and around the Aegean Sea. The former Byzantine lands in the Balkans were divided between the new Kingdom of Serbia, the Second Bulgarian Empire and the city-state of Venice. The power of the Byzantine emperors was threatened by a new Turkish tribe, the Ottomans, who established themselves in Anatolia in the 13th century and steadily expanded throughout the 14th century. The Ottomans expanded into Europe, reducing Bulgaria to a vassal state by 1366 and taking over Serbia after its defeat at the Battle of Kosovo in 1389. Western Europeans rallied to the plight of the Christians in the Balkans and declared a new crusade in 1396; a great army was sent to the Balkans, where it was defeated at the Battle of Nicopolis. Constantinople was finally captured by the Ottomans in 1453.
Output (English): In what year did Bulgaria become a vassal of the Ottomans?