In 1237-1241 Russian lands underwent a fierce attack from the Mongolian Empire - central Asian state, that conquered enormous territory of Euroasian continent from the Pacific ocean to Central Europe in the first part of XIII century. Mongols were called Tatars in Europe, by the name of one tribe that led nomadic life near the border with China. Chinese extended the name on all the Mongolian tribes, and the name "Tatars" defining Mongols extended to other countries, though Tatars were almost completely destroyed at the creation of the Mongolian Empire.
In 1206 Temujin (Temuchin) who accepted the name of Chingiskhan, was proclaimed the great Khan of all Mongols. During next 5 years Mongolian forces, united by Chingiskhan, conquered neighbouring lands and by 1215 had conquered North China. In 1221 Chingiskhan's army conquered Middle Asia.
Battle at Kalka.
First conflict between ancient Russia and Mongols was in 1223, when Mongolian army of 30.000 people came to Black sea steppes after defeating Alans and Polovtsy. Polovtsy asked Russian princes for help. And the united army of the three strongest Russian princes: Mstislav Romanovich of Kiev, Mstislav Svyatoslavich of Chernigov and Mstislav Metislavich of Galich, began the campaign against Mongolian tribes.
On May 31, 1223 at the battle on Kalka river (near the Azov sea) Russian-Polovtsy army was defeated because of uncoordinated actions. Six Russian princes were killed, three princes (including the prince of Kiev) were captured and then brutally killed. Thus, Russia for the first time experienced the military power of Mongolian Horde.
After the death of Chingiskhan, the founder of the Mongolian Empire (1227), Mongolian nobility decided to begin aggressive campaign against Europe according to Chingiskhan's will. Baty-khan, the grandson of Chingiskhan, headed the united army of the Mongolian empire. His first commander was Subedey, the outstanding Mongolian military leader who participated in the battle at Kalka.
Campaign to Northeast Russia (1237-1238)
A year after the beginning of the campaign, on conquering Volga Bulgaria, Polovtsy, lands of Burtas and Mordovia on the middle Volga in the late autumn of 1237, Baty's basic forces concentrated on Voronezh river preparing the invasion on Northeast Russia. Baty's horde consisted of 140.000 soldiers, and Russian princes could gather only 100.000 people from the whole Russian land, princes of Northeast Russia could gather not more than 30.000 people.
Internal wars that had been going on for decades hampered the formation of united Russian army. So, princes could resist the invasion only one by one. In the winter of 1237, Baty's horde ruined Ryazan principality, burned the capital and killed all the population. Then, in January of 1238, Mongols defeated the army of Vladimir-Suzdal land, captured Moscow and Suzdal. On February 7 they captured Vladimir. On March 4, 1238, on Siti river (upper Volga) they defeated the army of great prince Yury Vsevolodich who was killed in the battle.
The inhabitants of Kozelsk, a small town on Zhizdra river, did really a heroic feat of arms: for 7 weeks they managed to have been fighting with Mongols supporting the defense of their town. But the forces were obviously not equal. In May of 1238 Baty captured the town and in anger ordered to raze the town to the ground and kill all the people.
The summer of 1238 Baty spent in steppes near Don, restoring strength for future campaigns. In spring of 1239 he destroyed Pereyaslavl principality, in autumn - Chernigov-Seversk land.
Conquest of South Russia (1240-1241)
In the autumn of 1240 Baty's army went to Europe through the south Russia. In September they surrended Kiev where Galich prince Daniil Romanovich reigned. South Russian princes still hadn't managed to organize the defence of their lands from Mongolian threat. After severe struggle Kiev was captured in 1240. Then Mongol Horde ruined almost all towns of south Russia in December of 1240 - January of 1241.
In the spring of 1241 conquering the Galich-Volyn land, Baty entered Poland, Hungary, Czechia, and then came to the borders of North Italy and Germany. But the army didn't get reinforcment, had significant losses and in the end of 1242 had to return to the steppes of low Volga. here they founded the westmost ulus of the Mongolian Empire - the so called Golden Horde.