Emperor Stefan Uroš IV Dušan "the Mighty" (r. 1331–55) was succeeded by his son Stefan Uroš V "the Weak" (r. 1355–71), whose reign was characterized by the decline of central power and the rise of numerous virtually independent principalities; this period is known as the fall of the Serbian Empire. Uroš V was neither able to sustain the great empire created by his father, nor repulse foreign threats and limit the independence of the nobility; he died childless on 4 December 1371, after much of the Serbian nobility had been destroyed by the Ottomans in the Battle of Maritsa earlier that year. Prince Lazar, ruler of the northern part of the former empire (of Moravian Serbia), was aware of the Ottoman threat and began diplomatic and military preparations for a campaign against them.