Wallachian support of the Bulgarians south of the Danube who were fighting against the Turks brought them into conflict with the Ottoman Empire. In 1394, Bayezid I crossed the River Danube leading 40,000 men, an impressive force at the time, to attack Wallachia, ruled at that time by Mircea the Elder. Mircea had only about 10,000 men so he could not survive an open fight. He chose to fight what would now be called a guerrilla war, by starving the opposing army and using small, localized attacks and retreats (a typical form of asymmetric warfare). The Ottomans were superior in number, but 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.