Dmitry became Grand Prince of Moscow at nine. Metropolitan Alexis governed for the boy, and in these years Moscow replaced the wooden palisade of its Kremlin with white stone walls, which turned back several Lithuanian attacks.
In 1378 at the Vozha River he struck Mongol cavalry as they lost formation crossing the water and won — the first time a Rus' army had beaten a Horde force in the open field. Two years later at Kulikovo he chose a narrow field between rivers to deny the enemy horsemen room to flank him, and committed a reserve hidden in the woods at the decisive moment to turn the battle.
Tradition holds that before setting out he sought the blessing of the monk Sergius of Radonezh, a scene that became a favourite subject of Russian painting and literature. Mongol overlordship did not end that day — Tokhtamysh burned Moscow in 1382 — but the memory that the Rus' princes could win by acting together remained.