The Ottoman Turks under Bayazid I were becoming a much greater threat during Helen's rule than during the reigns of Dabiša and Tvrtko I, helped by their decisive victory over the Serbian lord Vuk Branković, whose land had stood between Helen's and Bayazid's. The Ottoman army that arrived in Bosnia in January 1398, led by Bayazid's sons and the subjugated Serbian lord Stefan Lazarević, was larger than those defeated by Helen's predecessors in 1388 and 1392. The plundering expedition failed completely, however, due to a severe winter and deep snow in which many of Bayazid's soldiers perished.