In fact, as the Mongols' position weakened in Russia in the latter half of the 14th century, one of Moscow's princes, Dmitry Donskoy, made war on them and inflicted the first major defeat of Mongols in Russia at the Battle of Kulikovo Field. This showed that the Mongols weren't invincible, which is always really bad for an imperial force. Plus, it made Moscow look like the hero of the Russians. And that helped strengthen the idea of a unified Russia, just as you'll remember the Persians helped unify the Greeks a long time ago. Aiding this growth was stability, which Moscow owed largely to luck. Muscovite princes usually had sons, which allowed them to have successors. In fact, there was only one major succession struggle, and it was between two blind guys named Basil.