Dmitry III "Donskoi" was the
son of Grand Prince
Ivan II of
Russia; he came to the
throne at the age of nine on his father's death in
1359. There was actually some dispute over whether he or his
uncle (Dmitry of Suzdal) should become Grand Prince, since it was quite common in Russia for a brother of the Grand Prince to succeed him rather than a son, and Dmitry of Moscow's age was a strike against him. The Mongol overlords of Russia were going through a period of civil strife (they had 20 Khans in 20 years) and different Mongols gave the Grand Prince's title to each of the contending Dmitrys. However, the support of the Russian
Orthodox Church and the people of Moscow won the day for the young Dmitry, whose claim became firm about four years after his father's death.
Church leader Metropolitan Alexis had been Ivan's advisor and was now essentially Dmitri's regent until 1367. They continued to expand the land ruled directly by Moscow, and also fortified the city with stone walls rather than wooden ones around the Kremlin. People were sought to settle in the empty parts of Muscovite land, as well as the eastern parts of Russia (such as the newer city of Nizhni Novgorod) and expand Russian territory; the Mongol internal problems gave the Russians a chance to do this.
By the time Dmitry took over as actual leader, Russia had some internal strife to deal with. Western cities such as Novgorod were in danger of Lithuanian conquest, and Prince Michael of Tver, with Lithuanian and sometimes Mongol backing, tried to increase his power at the expense of Dmitry's Moscow. In fact, Michael was named Grand Prince by the Mongols at one point, though he never had the practical power of the position. His Lithuanian friends reached the outskirts of Moscow in 1368 and 1372 but their troops were never able to take the city. (When the Lithuanians retreated, Moscow troops went to devastate the land around Tver in revenge.) Dmitry was able to persuade the Khan to reinstate him as official Grand Prince and give him Michael's son as a hostage; the Lithuanians backed down and Michael was forced to make peace with Dmitri.
But Dmitri is best known (and nicknamed for) the battle that took place September 7, 1380 in the Don River Valley. The Mongols had been attacking eastern Russian settlements for a few years, and had allied themselves with the Lithuanians. Forces from both side were separately on there way to Moscow, and Dmitri's troops went out to catch the Mongols farther away from the city and in a hilly area where their horsemen were less effective. The Battle of Kulikovo Pole was long and very close; it was only through the last-minute addition of some Russian reserves that the exhausted Mongols were defeated. This was a huge move for Russia against a hundred and forty years of Mongol rule, and suddenly a lot more princes supported Dmitry. The Lithuanians chose not to fight the Russians alone.
Within two years the Mongols were able to sack Moscow; the battles weren't over yet. There were also some Russian cities (especially Tver and Riazan) ruled by princes who were still against Dmitry. The last few years of his reign were not all that peaceful, but the Mongols were now just adversaries rather than rulers; Dmitri's reign had been a turning point in Russian history. He died relatively young in 1389 and was succeeded by his son Vasily I, although he had sown the seeds of future fighting by leaving a will promising that his second son Yuri would be Grand Prince after Vasily, rather than any son of Vasily's.