empire to disintegrate. The Khwarezmian Sultan Ala ad-Din Muhammad succumbed to disease on an island in the Caspian Sea, leaving his son, Jalal ad-Din Mingburnu landless. 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 Persian Iraq (Iraq-i Ajam) and Azerbaijan, sacking the cities of Rey, 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, which were the Mongols' best weapons. 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. Caucasus raid At the same time, Jebe's and Subutai's attention had turned elsewhere. In January and February 1221, they made a reconnaissance into the Kingdom of Georgia, entering through the Kura River. The goal of the Mongols was not to conquer the country but to plunder it, and the Kurds and Turcoman freebooters were sent off in the vanguard. However, the King of Georgia, George IV Lasha, advanced with 10,000 men and drove the Mongols back near Tbilisi. The Mongols withdrew, but continued to launch counter-attacks on the Georgian army. In March 1221, the Mongols returned to Azerbaijan and besieged Maragheh, using prisoners as the vanguard to take the brunt of each assault on the city. 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 defenders 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