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France and the Ottoman Empire, united by mutual opposition to Habsburg rule, became strong allies. The French conquests of Nice (1543) and Corsica (1553) occurred as a joint venture between the forces of the French king Francis I and Suleiman, and were commanded by the Ottoman admirals Barbarossa Hayreddin Pasha and Turgut Reis. A month prior to the siege of Nice, France supported the Ottomans with an artillery unit during the 1543 Ottoman conquest of Esztergom in northern Hungary. After further advances by the Turks, the Habsburg ruler Ferdinand officially recognized Ottoman ascendancy in Hungary in 1547.

France and the Ottoman Empire united against what?
Habsburg rule