Büyük Selçukluların yıkılmasından sonra güçlenerek kısa zamanda büyük bir imparatorluk haline gelen Hârzemşâhlar ile Karakorum'daki kurultaydan sonra dört bir yana genişleme kararı alan Moğollar kısa zamanda birbirlerinin askerî ve siyâsî rakipleri haline geldiler. Horâsân'da çok hızlı bir şekilde devam eden Moğol ilerleyişi kısa zamanda bu iki devleti karşı karşıya getirdi. Otrar Fâciası'ndan sonra derinleşen düşmanlıklar savaşı kaçınılmaz hale getirdi. Merv, Belh, İsfahan, Semerkand ve Herât gibi Horâsân'ın büyük şehirleri hızlı bir şekilde Moğolların işgaline uğradı ve halkı kılıçtan geçirildi. Moğollara karşı amansız mücadeleler veren Celâleddîn-i Hârzemşâh, Herât'ı da uzun bir direnişten sonra kaybetti. Herât bölgesinin birçok şehri kısa zamanda Moğolların eline geçti. Bu şehirlerin halkı kılıçtan geçirildi. Moğol ilerleyişi Celâleddîn Hârzemşâh'ı daha doğuya doğru çekilmeye zorladı. Celâleddîn'in doğuya doğru hızlı geri çekilmesi onun mücadele azminden vazgeçtiği anlamına gelmiyordu. Aksine bölgenin insanı onun direncini daha da arttırıyordu. Nitekim Herât ve Bâmîyân'ın kaybedilmesinden sonra Gazne-Bâmyân arasında Parvan denilen yerde Şigi Kutuku önderliğindeki Moğol ordusunu 1221 yılında ağır bir şekilde yenilgiye uğrattı. Bu zafer hem Celâleddîn-i Hârzemşâh'ın hem de devletinin adeta yeniden dirilişi olmuştur. Sultanın mücadele azmi arttığı gibi başta Herât ve Merv olmak üzere bölgenin birçok yerinde Moğollara karşı isyan hareketlerinde bulunmalarını da cesaretlendirmiştir

Khwarazmshahs that became an empire by becoming more and more powerful soon after the fall of Great Saljuks and Mongols that decided to expand everywhere around after the assembly in Kharkhorin became the military and political rivals of each other in a short time. The rapid Mongol invasion in Khurasan made these two states encounter in a short time. The enmity which deepened after the Otrar disaster rendered the war inevitable. The big cities of Khurasan such as Merv, Balkh, Ispahan, Samarkand and Herat were rapidly invaded by Mongols and people were massacred. Jalal-ad-Din Khwarazmshah who fought self-sacrificingly against Mongols lost Herat after a long resistance. Many cities of Herat region were conquered by Mongols. The people of these cities were massacred. The Mongol proceeding made Jalal-ad-Din Khwarazmshah withdraw to the further east. The rapid withdraw of Jalalad-Din to the east didn’t mean that he gave up his determination of fighting. On the contrary people of the region joined him because they thought he was the leader of liberation againist the Mongols. This case enhanced his resistence more. As a matter of fact, after the loss of Herat and Bamyan he heavly defeated Mongol army in the leadership of Sigi Kutuku in Parwan between Ghazni and Bamyan in 1221. This triumph became like the revival of both Jalal-ad-Din Khwarazmshah and his dynasty. Sultan’s determination of fighting increased and he also encouraged the people to rebel against Mongols in many parts of the region particulary in Herat and Marv.Khwarazmshah, who emerged as the successor of the Great Saljuk State after its collapse, became the most powerful political organization of the Islamic world in the beginning of XII. century in the south of Aral Lake, Ürgenç being the center. The borders of the state extended from the Sir-deryâ River to Azerbaijan and India. Emerging in the beginning of XIII. century in Turkistan, the Mongols, in a short time became an empire that involves the whole Asia, Eastern Europe, Turkistan, India and a large part of China thanks to governance of Genghis Khan. Genghis Khan wanted to get along well with Khwarazmshah at first. Because Khwarazmshah gave a very strong image of state at this time. However, the state was in a crisis because of its internal problems. Khwarazmshahs, although they had internal problems, were still a very important force. Khwarazmshahs were troubled that the Mongols had strengthened in a short time. The first event that confronts the two states is Otrar Disaster (Otrar Faciası). In 1218, the 450-person trade caravan sent by the Mongols was killed by Inalcik (Gayirhan) in Otrar. Despite this, Genghis Khan did not leave it at ease. He demanded the delivery of the criminals and the damages of the caravan to be compensated by the Khwarazmshah Sultan. However, he did not receive an answer to this request. Otrar Disaster became the turning point in the relationship of two states. Even if this disaster hadn’t happened, nobody would be convinced that Genghis Khan would remain on his seat and would not engage in conquest movements. But maybe the fight would not start so soon. Deepening hostilities after the disaster quickly confronted these two states. Genghis Khan began preparations for war. In 1218, the Mongols completed their preparations to a large extent and attacked the territory of Khwarazmshah. In a short time, they captured the important cities of Khwarazmshahs especially Otrar, Marv, Balkh, Bokhara, Nishapur and Herât and they looted, and they killed most of the people. Sultan Ala’uddin Muhammad, who failed to achieve success in the face of Mongol progression, escaped from Khurasan to Mazendaran and then to Abeskun in the Caspian Sea. A short time later, he died there on Shawwal 617/December 1220. Instead of Sultan Ala’uddin Muhammad, Sultan Jalal-ad-Din became the ruler. Sultan Jalal-ad-Din continued to fight with Mongols from the point his father left. When Sultan Jalal-ad-Din came to the place called Ustuva, he came across with Mongols here on Şadyah hill and defeated them. Sultan Jalalad-Din survived from the disaster on Transoxania and Khurasan by surpassing the Mongolian outpost line in Nesâ. The Sultan who came to Ghaznin was met here by Amînü'l-Mülk with 50 thousand soldiers. Seyfeddîn Igrak Kanglı and Khalaj Turks came to Ghaznin and joined to Sultan Jalaluddin. Many Muslims around who heard the arrival of the Sultan rushed to came under his order. Sultan Jalal-ad-Din gave a new dimension to the struggle with the Mongols with the army he established here in Ghaznin, by taking refuge in the middle of the mountains of Afghanistan. When he arrived in Parwan, he heard that the Mongol army under the command of the Tekecük and Molgar surrounded the fortress of Velian. Here, together with Amînü'l-Mülk, they defeated the Mongols. He took the fortress from them, too. Sultan Jalal-ad-Din who came victorious from this first clash, turned back to Parwan with the war booty he captured. Genghis Khan, who heard that 12 thousand people were defeated by Sultan Jalal-ad-Din, who had Ghaznin in his hand, sent a new army under the command of Şigi Kutuku, the famous Mongolian commander. The Mongolian army came to Parwan in the mid-morning a week after the arrival of Sultan Jalaluddin from Velian fortress to Parwan. Sultan Jalal-ad-Din's army got out of the city and embattled. Amînü’lMülk served on the right wing and Seyfeddîn-i İgrak on the left. The two armies fought here. Hârzemşâhlar won the fight, which was very tough and lasted two or three days according to the sources. After the war, the Kharzemshahs captured a great amount of booty and killed many Mongol soldiers. The exact date of the war was not found in the sources. However, it can be inferred that the war was in late April or early May of 618/1221 when considering the distance between Ghaznin and Parwan and that Juwani recorded that Khwarazmshah ruler moved to Parwan when the flowers blossomed. It should also be considered that the spring is late and the flowers will only blossom in these months because of the continental climate of the region. The Battle of Parwan is the heaviest defeat of the Mongols until this day. This victory earned against Mongols by Khwarazmshahs had a great impact on Muslims in the vicinity. After this victory of Sultan Jalal-adDin, the Muslims thought that Genghis Khan would not be able to take a stand against Sultan Jalaluddin. Battle of Parwan gave morale to the Khwarazmshahs. The state was almost resurrected. On the contrary, it increased the courage of the cities that survived on the east of Iran. For this reason, there was a riot against the Mongols in many places, especially in Marv and Herat. Genghis Khan, who was very angry with Parwan's defeat, ordered the destruction of the whole of Khorasan. Despite the victory of the Parwan War, Mongols destroyed Ghaznin and Talekan when the Sultan Jalal-ad-Din failed in resistance to the Mongols. They put the Herat people to the sword and burned even the ovarian wheat seeds of the people. They killed not only humans but also cats and dogs in Bamyan. Genghis Khan banned the establishment of another city here. Since the influence of victory could not be sustained, Khwarazmshahs soon collapsed, while the Muslims in the region were left alone with their own destiny.