Following the Mongol invasion of Central Asia and the subsequent collapse of the Khwarezmian Empire, a Mongol force under the command of generals Jebe and Subutai advanced into Iraq-i Ajam. Jebe requested permission from the Mongol leader, Genghis Khan, to continue his conquests for a few years before returning to the main army via the Caucasus. While waiting for Genghis Khan's reply, the duo set out on a raid in which they attacked Georgia and killed its king. Genghis Khan granted the duo permission to undertake their expedition, and after making their way through the Caucasus, they defeated a coalition of Caucasian tribes before defeating the Cumans. The Cuman Khan fled to the court of his son-in-law, Prince Mstislav the Bold of Galich, who he convinced to help fight the Mongols. Mstislav the Bold formed an alliance of the Rus' princes including Mstislav III of Kiev.
The combined Rus' army defeated the Mongol rearguard left to observe their movements. For several days, the Rus' pursued the Mongols but became spread out over a large distance. The Mongols stopped and assumed battle formation on the banks of the Kalka River. Mstislav the Bold, with his Cuman allies, attacked the Mongols without waiting for the rest of the Rus' army and were defeated. In the ensuing confusion, several other Rus' princes were defeated, and Mstislav of Kiev was forced to retreat to a fortified camp. After holding for three days, he surrendered in return for a promise of safe conduct for himself and his men. Once they surrendered, however, the Mongols slaughtered them and executed Mstislav of Kiev. Mstislav the Bold escaped, and the Mongols went back to Asia, where they joined Genghis Khan.
When Jebe (one of the Mongol generals pursuing Muhammad) heard of Ala ad-Din Muhammad's death, he asked Genghis Khan for a year or two to continue his conquests before returning to Mongolia via the Caucasus'.
While awaiting Genghis' reply, Jebe and Subutai (another general pursuing Muhammad) led their army of 20,000 men, with each general commanding a tumen. They left behind a trail of destruction as they moved through Iraq-i Ajam and Azerbaijan, sacking the cities of Rayy, Zanjan and Qazvin. The city of Hamadan surrendered without a struggle. Meanwhile, Özbeg, the Atabeg of Azerbaijan, saved his capital, Tabriz, and prevented his country's destruction by offering to the Mongols a large amount of money, clothing and horses.
From Tabriz, the Mongols advanced north and made their winter base in the Mugan Steppes. There, the army was strengthened by the arrival of Kurdish and Turcoman freebooters, who offered their services to the Mongols.
In March 1221, the Mongols returned to Azerbaijan and besieged Maragheh, using prisoners as cannon-fodder. By the end of the month, they had captured the city and put most of the population to death. Jebe and Subutai planned to advance south and capture Baghdad, the capital of the Abbasid Caliphate, and hold it for ransom while the Caliph was in Iraq-i Ajam with a small army. Instead, the Mongols turned once again to Hamadan. This time, however, the city's leaders failed to surrender, and its protectors inflicted many casualties upon the Mongol forces before their capture and plunder of the city.
In late 1221, the Mongols advanced north into Georgia once again, entering through the Kura River. A Georgian army was waiting near Tbilisi, and when Subutai advanced, he feigned retreat. At this point, the Georgian cavalry followed Subutai's army into an ambush set by Jebe. The Georgian army suffered a heavy defeat, and King George was mortally wounded. The Mongols proceeded to plunder southern Georgia.
Genghis Khan eventually granted Jebe permission and with Subutai as his second-in-command, the Mongols advanced to the city of Derbent, which refused to surrender. Jebe promised to spare the city in return for the services of 10 guides to take them through the Caucasus. To warn the guide against playing any tricks, the Mongols executed one of them. The crossing of the Caucasus was costly for the Mongols, who had to abandon their siege engines and lost hundreds of men to the cold.
After making it through the Caucasus, the Mongols were met by an alliance consisting of the Lezgians, the Alans and the Cherkesses, tribes who were living north of the Caucasus who had mustered an army of around 50,000 men. They were joined by the Cumans, a Turkish people who owned an expansive khanate stretching from Lake Balkhash to the Black Sea. The Cumans also convinced the Volga Bulgars and Khazars to join. The Cuman Khan, Koten, placed his army under the command of his brother, Yuri, and his son, Daniel. The first battle between the league and the Mongols was indecisive, but the Mongols managed to persuade the Cuman to abandon the alliance by reminding them of the Turkish-Mongol friendship and promising them a share of the booty gained from the Caucasian tribes.
With this arrangement settled, the Mongols attacked the alliance's army and routed it. The Mongols then proceeded to attack the Cumans, who had split into two separate groups as they were returning home, destroying both armies and executing all the prisoners before sacking Astrakhan. The Mongols began pursuing the Cumans as they fled in a north-westerly direction.
The Venetians sent a delegation to the Mongols, and they concluded an alliance in which it was agreed that the Mongols would destroy any other European trading post they came across. As the Mongols pursued the Cumans, Jebe sent a detachment to Crimea, where the Republic of Genoa had trading stations. The Mongols captured and plundered the Genoese city of Soldaia. Meanwhile, Koten fled to the court of his son-in-law, Prince Mstislav the Bold of Galich. He warned Mstislav: "Today the Mongols have taken our land and tomorrow they will take yours". However, the Cumans were ignored for almost a year as the Russians had suffered from Cumans raids for decades. But when news reached Kiev that the Mongols were marching along the Dniester River, the Russians responded. Mstislav gathered an alliance of the Kievan Rus' princes including Mstislav III of Kiev and Prince Yuri II of Vladimir-Suzdal, who promised support. The Rus princes then began mustering their armies and going towards the rendezvous point.
The number of men present at the battle is disputed. A major factor in this is the fact that no primary sources give the number of men present at the battle, which leaves modern historians to estimate the number of men. Historian Leo de Hartog gives the size of the Rus' army as 30,000, while Richard Gabriel and Hector Hugh Munro claim that the size of the Rus' army was 80,000. de Hartog also estimates the size of the Mongol army as 20,000, while Gabriel estimates that it was around 23,000 men. However, historian John Fennell, an expert on Kievan Rus and early Russian history, and one well-versed in the primary sources, calls many of these figures into doubt, saying the numbers given in the Russian sources (there are no Mongol or Polovtsian sources, at least none that have survived, and other sources from other cultures, if they exist, are rather dubious) are formulaic or exaggerated and the chronicles contradict themselves.
The move by the Rus' army was detected by the Mongols, who were on the east side of the Dnieper River waiting for reinforcements from Jochi, Genghis Khan's eldest son, who was campaigning around the Aral Sea. Jochi, however, had become ill, which meant no reinforcements would be coming.
At the same time, the Rus' attempted to trap the Mongols. The Princes of Galich and Volhynia transported their armies south down the river, while the Princes of Kiev and Chernigov advanced north up the river, and the army of Kursk advanced from the front. At the same time, the Cumans attempted to attack the Mongol army's rear. When Jebe learned of this, he sent 10 envoys to the Prince of Kiev. The envoys stated that the Mongols had no feud with Rus and were only attacking the Cumans; they added that the Mongols were marching east, away from the Rus' cities. Mstislav of Kiev had the envoys executed, and the Mongols responded by sending another set of ambassadors, who declared war.
When Jebe and Subutai heard of the Rus' movements, they began moving east, away from Rus', which was the only direction in which they could move. However, they left a rearguard of 1,000 under the command of an officer, Hamabek, to report of the Rus movements. Soon, Mstislav the Bold, reached the river opposite the rearguard, and it became apparent that no Prince had been appointed commander-in-chief. Thus, all the Princes could act as they pleased. Eventually, Mstislav crossed the river under heavy arrow fire. When the Rus' landed, however, their numbers were too great, and the Mongols were killed to the man.
After drawing out the Rus armies for nine days in a feigned retreat, the Mongol army turned to face their pursuers along the Kalka River (the river's location is currently unknown, but it is thought to be the Kalchik River flows into the Sea of Azov.
The Russian primary sources give only a very general account of the battle itself and the pursuit of the princes back across the steppe. The chronicles name which princes took part and which died, but not much more in terms of the size of armies or casualties. As to the actual battle itself, the chronicles report that the Polovtsy broke and ran without having fought and that their flight through the Russian ranks led to mass confusion and resulted in their slaughter by the Mongols.
The armies of Volhynia and Kursk made a gap in their line so that the fleeing Cumans could retreat. However, the Mongol heavy cavalry charged through the newly formed gap. The army of Chernigov, which was not aware that the battle had started, was advancing when they collided head-on with the retreating Cumans. The Mongol cavalry took advantage of the confusion in the Chernigov line and attacked, causing the line to collapse. This, in turn, led to the death of Prince Mstislav of Chernigov.
At the same time, the Mongol wings closed around the shattered Rus' army, cutting off its retreat. The surrounded Rus' were hit by volley after volley, accompanied by occasional cavalry charges. As the Mongols were carrying this annihilation out, some of the army – led by Mstislav the Bold – managed to cut their way through the Mongol ring and escape.
Mstislav of Kiev arrived to see what remained of the Rus' army fleeing. With his contingent of 10,000 men, he retreated to his stockaded camp, on a hill by the Dnieper. The pursuing Mongol army caught up with Mstislav of Kiev's forces and started to besiege the camp. Mstislav of Kiev and the Kievan army managed to hold out for three days, but Mstislav of Kiev decided to surrender to Jebe on the condition that he and his army would be able to return unharmed to Kiev. Once in control of the camp, the Mongols slaughtered the Kievan army and took Mstislav of Kiev and several other nobles prisoners.
The battle was a very costly defeat for the Rus' princes, with Richard Gabriel claiming that they lost 50,000 men, while the Mongol losses were minimal. Out of the Rus' primary sources, The Primary Chronicle gives a number of 10,000 killed while the much later (and much less reliable) Nikonian Chronicle cites 60,000 killed. The Novgorodian First Chronicle, contemporary to the battle, gives no figures at all. Out of the Rus' princes, the wounded Daniel of Volhynia and Mstislav the Bold managed to escape the battle. This battle was a significant defeat, given that many of the Rus principalities lost much of their armies, with the notable exception of Vladimir-Suzdal. Historian Robert Marshall describes the raid as follows: "The rest of Subedai's campaign has entered the annals of military history as one of the greatest adventures of cavalry warfare."
The Mongols executed Mstislav of Kiev and the Kievan nobles in the traditional Mongol manner reserved for royalty and nobility; without shedding blood. Mstislav and his nobles were buried and suffocated under the Mongol general's victory platform at the victory feast. Meanwhile, Mstislav the Bold managed to reach the western side and the Dnieper with what remained of his army. To stop the Mongols from crossing to the western side of the Dnieper, Mstislav destroyed all the boats he could find.
What the Rus' feared would happen did not as the Mongols pursued the Prince of Galich and plundered a few towns in the south before turning around. The Mongol army crossed the Volga River near modern-day Volgograd and passed through Volga Bulgaria, where they were defeated in an ambush by the Bulgars. The Mongol army encountered the Bulgars in another battle in which they routed the Bulgars. The Mongols followed this up by attacking the Kanglis Cumans, who had supported their fellow Cumans in the Caucasus a year before. They fought against the Cuman army near the Ural Mountains, defeating and killing the Khan before making them pay tribute.
Following this victory, the Mongols turned east and met Genghis Khan and the rest of the Mongol army in the steppes to the east of the Syr Darya River. Genghis Khan showed great appreciation for his general's achievements and heaped praise on Jebe and Subutai. Jebe, however, did not survive the campaign long; he died soon afterwards. The importance of the expedition was immense. The expedition was history's longest cavalry raid, with the Mongols riding in three years. Subutai also stationed numerous spies in Russia, who provided frequent reports on what was happening in Europe and Russia. In 1237, Subutai together with Batu led another attack on Rus, this time with 120,000 men and with this army, he conquered Kievan Rus'.