From 1389 to 1395 he conquered Bulgaria and northern Greece. In 1394 Bayezid crossed the River Danube to attack Wallachia, ruled at that time by Mircea the Elder. The Ottomans were superior in number, but on 10 October 1394 (or 17 May 1395), in the Battle of Rovine, on forested and swampy terrain, the Wallachians won the fierce battle and prevented Bayezid's army from advancing beyond the Danube.