The most important event during Demetrios's early reign was to start building the Moscow Kremlin; it was completed in 1367. Thanks to the new fortress, the city withstood two sieges by Algirdas of Lithuania during the Lithuanian–Muscovite War (1368–72).[3] The war ended with the Treaty of Lyubutsk. In 1375, Demetrios settled, in his own favor, a conflict with Mikhail II of Tver over Vladimir. Other princes of Northern Russia acknowledged his authority and contributed troops to the impending struggle against the Horde. By the end of his reign, Demetrios had more than doubled the territory of the Principality of Moscow.
Mongol domination of Rus began to crumble during Demetrios's thirty-year reign. The Golden Horde was severely weakened by civil war and dynastic rivalries. Demetrios took advantage of this lapse in Mongol authority to openly challenge the Tatars. While he kept the Khan's patent to collect taxes for all of Russia, Demetrios is also famous for leading the first Russian military victory over the Mongols. Mamai, a Mongol general and claimant to the throne, tried to punish Demetrios for attempting to increase his power. In 1378 Mamai sent a Mongol army, but it was defeated by Demetrios's forces in the Battle of Vozha River.
As they made camp before the Battle of Kulikovo, an icon of Saint Nicholas appeared in the air, hovering over a pine tree, and it descended into the hands of Prince Demetrios. There is a later icon depicting this event, with Saint Demetrios kneeling before the icon of Saint Nicholas, and laying his gold crown at the roots of the tree.
After winning the battle, the Prince ordered a Moleben of Thanksgiving to God and to Saint Nicholas to be served. Later, he built a church and a monastery dedicated to Saint Nicholas on that site.
Following his victory at Kulikovo Field, between the Don and Nepryadva Rivers (on September 8, the Great Feast of the Nativity of the Most Holy Theotokos), Prince Demetrios received the honorific "of the Don." He established the Dormition Monastery at the Dubenka River, and the Church of the Nativity of the Most Holy Theotokos near the graves of those who died for their country. The Memorial Saturday before the Feast of Saint Demetrios of Thessalonika (October 26) was established in memory of the Orthodox warriors who were killed at Kulikovo Field in the great battle against the Horde.
Saint Demetrios fell asleep in the Lord on May 19, 1389. He was buried in the Cathedral of the Archangels in the Moscow Kremlin. He was the first Great Prince to bequeath his titles to his son Basil I of Russia without consulting the Khan.
In you the Russian land has found a great defender in a time of tribulation, O vanquisher of the pagans. At the Don you defeated the vainglorious Mamai; having received the blessing of Saint Sergius for this undertaking; and so, Prince Demetrios, pray to Christ God to grant us great mercy.
Kontakion in the Second Tone
By your deeds, Saint Demetrios, God preserved our country by granting you invincible power. And now, our powerful defender, by your holy prayers, keep your city of Moscow safe from all hostile schemes.




