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In 1445 King Kniaz Rurovich of Tver dies without an heir, Muscovy (whom shares the same dynasty as Tver) inherits the throne and the two countries form a Personal Union. Lithuania still has some independence from Poland and thus attacks Muscovy over the throne of Tver.
The resulting war absolutely destroys Muscovy's army and morale. The Golden Horde and Kazan eventually invade too. The Lithuanian peace treaty with Moscow essentially breaks up the would-be-empire as Perm and Novgorod gain lands. In the south the Hordes gain land and prestige.
Lithuania eventually falls under Poland's control and thus retires from the diplomatic scene in Russia. The Islamic Hordes are now once again the dominant powers with enough manpower to overcome the more advanced Russians.