evidence: In the course of centuries, various parts of present-day Kazakhstan were incorporated into different empires. During the empire of the Mongols (13th–14th century ce), most of the territory was part of the ulus ("polity") of Chagatai. About 1465, under the leadership of Karay and Jani Beg, some 200,000 dissatisfied subjects of the Uzbek khan Abū’l-Khayr (Abū al-Khayr) moved into Mughulistān, whose khan, Esen Bogha (Buga), settled them between the Chu and Talas rivers. These separatist Uzbeks became known as Kazakh ("Independent" or "Vagabond") Uzbeks, and over time a significant differentiation developed between them and the nonseparatist Uzbeks in their respective ways of life: that of the Kazakhs was more nomadic, that of the Uzbeks more sedentary. [...] Kazakhstan formally became a constituent (union) republic of the Soviet Union on December 5, 1936. During the first secretaryship of Nikita Khrushchev, the role of Kazakhstan within the Soviet Union increased dramatically. The Virgin and Idle Lands program launched in 1953 opened up the vast grasslands of northern Kazakhstan to wheat farming by Slavic settlers, a program that, over the course of several decades, led to an ecological disaster (see Aral Sea). Kazakhstan’s significance in the Soviet period also increased through the location on its territory of the main Soviet space-launch centre and a substantial part of the Soviet Union’s nuclear weaponry and the sites associated with nuclear testing. [...] Presidency of Kassym-Jomart Tokayev (2019– )