Vasily II was the son of Grand Prince
Vasily I of Russia, who became Grand Prince at the age of 10 on his father's death in
1425. His right to the throne was contested by his uncle Yuri and later Yuri's sons Dmitry and Vasily the Squint-Eyed. Vasily I's father
Dmitry III had left a will saying that Yuri should rule after Vasily I, and Yuri meant to make sure that was carried out. Vasily II's supporters claimed that Dmitry III's will assumed Vasily I would die childless, as he was at the time of his father's death.
Even though the Russians were trying to slide out from under the control of the Mongols, both claimants went to Khan Ulu Mohammed to plead their case and try to get him to pick a Grand Prince. The Khan upheld the succession from father to son and chose Vasily; at first Yuri and his supporters accepted this, but later grew to feel they had been slighted and rebelled.
Yuri's supporters managed to capture Vasily; they banished him to Kolymna. However, Vasily's supporters came to his aid, and Yuri was temporarily induced to restore Vasily when Yuri's sons rebelled. Their rebellion managed to succeed, though, and Vasily was banished again to Nizhni Novgorod. Yuri died in 1434 and his sons fought among themselves, so Vasily had more power than anyone else.
Soon, the Mongols started rebuildling Bulgar cities on the Volga River and making raids into Russian territory. In 1445, Moscow was attacked, and the city of Suzdal was destroyed. Vasily paid a large ransom for Russian prisoners of war, which was not a popular move; the next year he was surprised while in church, captured by Russian enemies including Yuri's sons, and blinded. (Henceforth he was known as Vasily the Dark or the Blind.) However, he had enough supporters that by the next year he was restored to this throne, and by 1450 the last revolt from Yuri's sons was put down.
After this, the Mongols became the larger problem. Some of their princes had supported Vasily in the civil war, and he rewarded them with Russian estates and titles. Others preferred to try and win back their overlordship of Russia (especially as the Ottoman Turks were encroaching on their southern frontiers, and the Mongol government was dealing with various claimants to the throne; during this period it would break into several pieces). Khan Ahmed sent attacks on Moscow in 1451, 1455, and 1461 but never managed to do permanent damage to Russia. In 1456, Moscow sent troops to Novgorod because of its support of Yuri's son Dmitry; due to Muscovite mounted archers they won and were able to force on Novgorod a treaty that made it much more subject to Moscow than it had been.
Vasily the Dark died in 1461 and was succeeded by his son Ivan III.