Excerpts from Books and Wikipedia
"After Alexander Nevsky, the region once again broke up into petty states, though the Grand Duchy of Moscow, founded by Alexander Nevsky's youngest son Daniel, began to consolidate control over the entire Rus' territory in the 15th century, starting the story of Russia. Following the Mongol conquests of the 13th century, all of the Russian principalities paid tribute to the Golden Horde, effectively operating as vassals of the Mongol state. The Russians began to exert independence from the Mongols, culminating with Ivan the Great of Moscow ceasing tribute to the Horde, effectively declaring his independence. His son Vasili III completed the task of uniting all of Russia by eliminating the last few independent states in the 1520s." [Wikipedia]