In 1359, when Prince Dmitry Ivanovich was almost 10 years old, his father Grand Duke Ivan the Good of Moscow died, leaving the young Prince as the new Grand Duke. Metropolitan Alexis of Moscow, who would later be canonized a saint, was in charge of educating the young Grand Duke. Through Metropolitan Alexis, Dmitry learned that his main goal would be the unification of Russia and liberation from the Mongol-Tartar yoke.
As Dmitry grew up the Tartars stopped attacking Russia; content with the tribute they were collecting, and this fostered a courage and boldness in Dmitry. So that, when the Horde became noticeably weaker, Dmitry began to strongly focus on the unification and subordination of the unruly Russian cities to Moscow for he understood that salvation of Russia would only come from unity.
Grand Duke Dmitry Ioannovich Donskoy
In 1368 Dmitry began to annex many cities to Moscow, some resisted, but many fell in under the Muscovite banner including; Vladimir, Suzdal, Rostov, Yaroslavl, Kashin, Starodub, Obolensk, Tarusa, and Novosil. The notable cities against Moscow were Tver, Smolensk, and Ryzan, even bringing in Lithuanian armies to campaign against Dmitry.
By this time the Khan of the Mongol-Tartar Horde noticed and disliked the fact that Moscow was no longer as submissive as before, so he gathered his army, hired mercenaries, allied himself with the Lithuanian king, and marched on Moscow.
On learning of this, Dmitry rode to the Trinity-Sergius Lavra to get a blessing and spiritual support from the Venerable Sergius of Radonezh, later St. Sergius of Radonezh. Along with his blessing and a prediction of victory, Sergius also gave Dmitry two of his monks who in the past had been famous warriors, Alexander Peresvet and Andrey Oslyabya. Because of this, many cities joined in defending Moscow; excluding Tver, Suzdal, and Ryzan.
In marching towards the Horde’s armies, two miracles occurred for St. Dmitry and the Muscovite army. First, the Lithuanian princes, the younger brothers of the Lithuanian king, decided to join Moscow and fight against their brother. This was because the Lithuanian king wanted to unite with Poland, and the princes wanted to remain independent. Second, the atamans of the Don Cossacks, experienced generals and warriors, also joined Dmitry to oppose the Horde.
Even at that though Dmitry and his troups were still outnumbered by the Khan’s armies. But they pushed forward anyway and using the examples of Yaroslav the Wise and Alexander Nevsky, once they crossed the Nepryadva River they burnt the bridges behind them. This meant that the soldiers were ready to stand to the death as they now had nowhere to retreat. They lined up in Kulikovo Field, for which the battle was later called and faced the Mongol Horde.
Blessing of Dmitry Donskoy by the Venerable Sergius of Radonezh
Before the battle Dmitry rode around all the regiments and encouraged them. Some of the regiments lined up directly with the enemy, but some were hidden in a nearby grove as an ambush. Early in the morning, the Tartars charged. The first to fall were the valiant monks Peresvet and Oslyabya, and then Mikhail Bryanko, an important general boyar. It seemed at first like the Horde was winning, but finally at the ninth hour when the wind was blowing favorably the ambush regiment struck, scattering the Horde in confusion and terror. As so, even at a terrible price, the victory was won and Russia was both unified and liberated as the Metropolitan had predicted to Dmitry long ago. This victory is still celebrated today with the Orthodox Church commemorating those who fell in this battle on Holy Saturday.
Source: Pokrov (Russian)