After the battle of Nicopolis, the Ottomans occupied Bulgaria and could attack Wallachia more easily. Mircea I the Old, however, could reoccupy Dobruja in 1402 by taking advantage of the Ottomans' difficulties after their defeat by Timur Lenk in the battle of Ankara. He even intervened in the Ottoman civil war and supported the struggle of Musa and Mustafa against their brother, Mehmed I. After the two pretenders had been defeated, the Ottomans annexed again Dobruja and occupied Giurgiu, and Mircea I was forced to pay an annual tribute to the sultan. Under Mircea I iron mines were opened at Baia de Fier and copper mining began at Baia de Aramă. In addition, sulfur and amber were extracted in the region of Buzău.