"An Historical Atlas of Central Asia" written by Yuri Bregel. This is stated on Page no 37 of this book.

At a quriltay (an assembly of Mongol princes) held in northern Mongolia in 1206 Chingis Khan was proclaimed the supreme ruler of all the Mongols. By 1207-1209 he had already subjugated the Tanguts, who lived in the Ordos region, and in 1211 the Mongols invaded northern China.
In 1215 they captured Beijing, and in 1216, with most of Northern China in their hands, Chingis Khan returned to Mongolia. Possibly soon after this (although it could have happened about two years later) the Mongols under Subuday and Taghachar had their first encounter with the troops of the Khorezmshah Muhammad in the Dasht-i Qïpchaq (see map 17). In 1215 Muhammad sent an embassy to Chingis Khan, who sent in return his embassy
to the Khorezmshah in 1218; at the same time a large trade caravan of Muslim merchants set out from Mongolia and reached Otrar after Chingis Khan’s ambassador already returned. The governor of Otrar, named Inalchik, had all the 450 or so merchants of the caravan slaughtered and their goods confiscated, accusing them of being spies (the massacre seems to have been sanctioned by the Khorezmshah himself). An ambassador of Chingis Khan came to the Khorezmshah soon after this, demanding the extradition of Inalchik, but he was also killed. After this war became inevitable.
For some time before this, the Mongol Empire had become an immediate neighbor of the Khorezmshah’s empire: while Küchlük was taking possession of the Qara-Khitay state, in 1211, the Qarluq ruler of Semirech’e, Arslan Khan, declared himself a subject of Chingis Khan; and at the same time a certain Buzar, a former robber, adopted the title of Toghrul Khan, founded his own principality centered in Almalïq, and also submitted to Chingis Khan, having sent to him his son, Suqnaq Tegin. Before his major campaign against the Khorezmshah, Chingis Khan sent troops under Jebe Noyon against Küchlük in 1218. Jebe went from Almalïq to Balasaghun, which surrendered without resistance, and then entered Kashgharia, where he issued an order returning to the Muslims the right to have public religious services; the population met the Mongols as liberators from the oppression of Küchlük. Küchlük was pursued by Jebe until the Sarïqol, where he was caught and killed; his severed head was shown in the cities throughout Eastern Turkestan.