A question from Yahoo! Answers:
What was the main reason the princes of Moscow were able to become the rulers of Russia?
Partly dumb luck, partly smart politicking.
In early 14th century, Moscow and Tver were jockeying for position of the regional leader. Prince Mikhail of Tver wanted an all-out war against the Horde, which earned him an assassination in 1318. His son Dmitry wanted to continue his father’s line and entered into an alliance with Lithuania.
Moscow (and its prince Ivan Kalita), meanwhile, remained a faithful vassal of the Horde (in fact, Moscow had familial ties with the Horde: Ivan’s older brother, Yuri, was Khan Uzbeg’s brother-in-law).
In 1326, Mongols assassinated Dmitry of Tver, and Tver responded by uprising. Ivan Kalita joined forces with the Horde and together the Horde and Moscow repressed the rebellion. There was no clear heir to Dmitry, so Tver’s leaders got mired in dynastic squabbles, and Tver gradually lost its influence. Moscow, meanwhile, grew in importance, because the Horde outsourced tax collection to it…
Another smart move by Ivan was that he talked Mitropolit (Archbishop) Peter, the leader of the Russian Orthodox Church, to move his seat from Vladimir, which was quickly losing its importance, to Moscow in 1325. So Moscow became the religious capital of Russia, which was doubly important for Ivan, since in 1313, Mitropolit Peter visited the Horde, where Khan Uzbeg granted him the privilege to be the sole judge of all Russian clergy.