The Battle of Varna in 1444 was arguably the most important Ottoman victory in Europe prior to their conquest of Constantinople, especially if you consider its effects alongside those of the (second) Battle of Kosovo in 1448. The Ottoman success at Varna shattered a Hungarian-Polish alliance that had been formed to counter the Turkish threat. That alliance was such a big deal that Pope Eugenius IV (d. 1447) gave it the Church’s official imprimatur, which is why you sometimes find the Battle of Varna discussed in the context of something called the “Crusade of Varna,” even though the “crusade” consisted basically of a couple of preliminary engagements followed by this one major battle. The end of the “crusade” and the alliance behind it had huge implications for Europe, in that it gave the Ottomans time to focus on Constantinople, which, despite the near-irrelevancy of the Byzantine Empire by the mid-15th century, was still the big prize.
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