Golden Horde
The Mongol khanate that ruled the Russian principalities for roughly 240 years, and in doing so built the state that became Muscovy.
The Golden Horde is the western wing of the Mongol empire, the share allotted to Genghis Khan's eldest son Jochi and conquered by Jochi's son Batu. Between 1237 and 1241 Batu's armies destroyed the Rus principalities one by one, burning Ryazan, Vladimir, and finally Kiev in December 1240, and rode on to defeat the Poles and Hungarians before turning back. What they left behind on the Russian steppe lasted, in one form or another, for the next two and a half centuries.
Rule by remote control
The Horde did not occupy the Russian lands. It taxed them. From its capital at Sarai on the lower Volga, the khan issued the yarlyk, the patent that authorized a Rus prince to rule his city, and a prince who wanted one had to travel to Sarai and grovel for it. Some never came home. The Horde also conducted censuses, the first systematic ones in Russian history, in order to assess tribute and conscript men.
This is the arrangement Russian historiography calls the Tatar yoke, and the standard nationalist reading treats it as a dark age imposed from outside. The more interesting reading is that Moscow was built by it. Moscow was an insignificant town in 1237. Its princes made themselves the khans' preferred tax farmers, collecting tribute from rival Russian cities and skimming the difference, informing on competitors, and using Mongol troops to crush them. Ivan I, remembered as Kalita, the moneybag, was the master of the technique. By the time the Horde weakened, Moscow had the treasury, the metropolitan of the Orthodox church, and the habit of command.
What the Mongols taught, it is often argued, went deeper than tax collection: the census, the postal relay system, the assumption that the ruler owns the realm and its people serve at his pleasure. Russian autocracy has more than one parent, but the Horde is a plausible one.
Islam, plague, and fracture
The Horde converted. Berke, Batu's brother, became a Muslim in the 1250s, the first major Mongol ruler to do so, and promptly went to war with his cousins ruling Persia, who had just sacked Baghdad; the Mongol empire's unity was already gone. Under Uzbeg Khan in the early fourteenth century Islam became the state religion, and the Horde grew rich on the Black Sea trade linking the Silk Road to Italian merchants at ports like Kaffa in Crimea.
That trade route also carried the Black Death west in the 1340s. Then came dynastic chaos, and in the 1390s the conqueror Timur invaded, sacked Sarai and the trading cities, and wrecked the economy the Horde ran on.
The political end was drawn out. Prince Dmitry of Moscow beat a Horde army at Kulikovo in 1380, a battle Russians have celebrated ever since, but Moscow was sacked in reprisal two years later and kept paying. Real independence came in 1480, when Ivan III faced a Horde army across the Ugra river, both sides stared at each other for weeks, and the Mongols went home. The Great Stand on the Ugra ended the tribute without a battle.
The long tail
The Horde broke into successor khanates at Kazan, Astrakhan, Sibir, and Crimea. Moscow, once the tax collector, spent the next three centuries eating them: Kazan in 1552, Astrakhan in 1556, Siberia after that. The Crimean Khanate survived longest under Ottoman protection, raiding Russian and Ukrainian territory for slaves until Catherine the Great annexed it in 1783.
That sequence is the beginning of Russian expansion as a permanent project. A state that spent 240 years paying tribute to steppe cavalry, on a plain with no natural defensive line, came out of the experience with a conviction it has never lost: security means controlling the ground the threat comes from, and the frontier is never far enough away.