What allowed Dmitry Donskoy to win a convincing victory over the Mongols?

He was allowed to do this: faith in his victory, faith in God, courage and steadfastness of his army, as well as military leadership. The battle itself was very significant. Two large armies met in the battle. The battle went on for a very long time, the Russian regiments were constantly pressed. It seemed that a little more and the Russian army would falter and the enemy would win. But at the last moment the ambush regiment struck, which managed to overturn the enemy and throw him into confusion. This is how the bloody battle on the Kulikovo field was won.