Öz Beg Khan Sultan Mohammed Öz Beg , better known as Uzbeg or Ozbeg ( 1282 – 1341 , reign 1313 – 1341 ) , was the longest - reigning khan of the Golden Horde , under whose rule the state reached its zenith . He was succeeded by his son Jani Beg . He was the son of Toghrilcha and grandson of Mengu - Timur , who had been khan of the Golden Horde from 1267 – 1280 . Coronation and Conversion to Islam by the Horde . Öz Beg ' s father Togrilcha was one of Genghisid prince s that overthrew Tode - Mengu ( r . 1280 – 1287 ) . Later , he was executed by Tokhta ( 1291 – 1312 ) . Tokhta took Togrilcha ' s wife and sent his son Öz Beg to exile in a distant region of the Golden Horde : either Khorazm or the country of Circassians . Converted to Islam by Ibn Abdul Hamid , a Sufi Bukharan sayyid and sheikh of the Yasavi order , Öz Beg assumed the throne upon the death of his uncle Tokhta in January 1313 with the help of the former Khans ' vizier Temur Qutlugh and of Bulaghan ( or Bayalun ) khatun . At first , many Mongol nobles were against him and organized a plot to kill the new khan . Öz Beg discovered the plot and crushed the rebels . His adoption of Islam as a state religion led to a conspiracy of Shamanist and Buddhist princes , which he subdued severely . Öz Beg determinedly spread Islam amongst the Golden Horde and allowed missionary activities to expand in the surrounding regions . Öz Beg found out that his competitor was backed by the envoys of the Great Khan Ayurbarwada Buyantu and this fact helped deteriorate his relationship with the Yuan Dynasty . The last of his rebellious relatives was shamanist Khan Ilbasan of the White Horde , who was murdered in 1320 . Öz Beg installed the Muslim Mubarak Khwaja as a replacement to the throne of the White Horde , but he discouraged their independence . In the long run , Islam enabled the Khan to eliminate inter factional struggles in the Horde and to stabilize state institutions . Russian scholar Lev Gumilev wrote that in this manner was Öz Beg able to turn the khanate into a sultanate . Khan Öz Beg urged the Mongol elite to convert to Islam , but at the same time , he preserved the lives of Christians and pagans such as Russians , Circassians , Alans , Finno - Ugric people , and Crimean Greeks as long as they continued to pay the jizyah in subjection to Islamic rule . From Öz Beg onwards , the khans of the Golden Horde were all Muslim . Öz Beg was very tolerant of Christians as exemplified by a letter of thanks he received from Pope John XXII in which the Christian leader thanked Öz Beg for his kind treatment of Christians . Öz Beg had sent a letter to the Metropolitan Peter which stated : By the will and power , the greatness and most high ! Let no man insult the metropolitan church of which Peter is head , or his service or his churchman ; let no man seize their property , goods or people , let no man meddle in the affairs of the church . . . Their laws , their churches and monasteries and chapels shall be respected ; whoever condemns or blames this religion , shall not be allowed to excuse himself under any pretext , but shall be punished with death . Reign . Military and Politics . Öz Beg maintained one of the largest armies in the world , which exceeded 300 , 000 warriors . He employed his military clout to conduct campaigns against the Ilkhanate in Azerbaijan in 1319 , 1325 and 1335 . Ilkhanid commander Chupan repulsed one Öz Beg ' s first two attempts and even invaded deep into the Jochid Ulus in 1325 . Öz Beg found an ally against the Ilkhanids in Mamluk Egypt ; indeed , one of Cairo ' s squares was named after him . The Khan had the daughter of previous Khan ' s sister , Princess Tulunbuya , married to a Mamluk sultan , but she died soon after and Öz Beg was disappointed . In 1323 , a peace treaty was signed between Egypt and the Ilkhanate . This situation nullified the alliance and the Mamluks refused to invade the Ilkhanate . Öz Beg ' s next incursion coincided with Abu Said ' s death . However , the weather turned bad and the new Ilkhan Arpa Ke ' un came with a large force ; Öz Beg ' s army was forced to withdraw . Chagatai Khan Esen Buqa I attempted to gain the support of Öz Beg Khan against Buyantu , the Great Khan of the Mongol Empire and the Emperor of the Yuan Dynasty , in 1313 and 1316 . Esen Buqa warned Öz Beg that the Great Khan would overthrow him from the throne of the Horde and install another Khan from the Jochids instead . But Öz Beg ' s vizier convinced him not to believe this and the Khan refused to help Esen Buqa . Nevertheless , remembering their support for the rival claimant to his throne , Öz Beg tried his best to eliminate every influence and inspiration of the Yuan Dynasty on the Golden Horde in the early part of his reign . The Khan ' s diplomatic relationship with the Yuan , however , improved in 1324 . By the 1330s , Öz Beg had begun sending tribute to the Mongol Yuan Emperors and received his share from Jochid possessions in China and Mongolia in exchange . Öz Beg was engaged in wars with Bulgaria and the Byzantine Empire from 1320 to 1332 . He repeatedly raided Thrace , partly in service of Bulgaria ' s war against both Byzantium and Serbia that began in 1319 . His armies pillaged Thrace for 40 days in 1324 and for 15 days in 1337 , taking 300 , 000 captives . After Öz Beg ' s death in 1341 , his successors did not continue his aggressive policy and contact with Bulgaria lapsed . His attempt to reassert Mongol control over Serbia was unsuccessful in 1330 . Byzantine Emperor Andronikos III purportedly gave his illegitimate daughter in marriage to Öz Beg but relations turned sour at the end of Andonikos ' s reign , and the Mongols mounted raids on Thrace between 1320 to 1324 until the Byzantine port of Vicina Macaria was occupied by the Mongols . Andonikos ' s daughter , who adopted the name Bayalun , managed to escape back to the Byzantine Empire , apparently fearing her forced conversion to Islam . Öz Beg allowed Genoese merchants and mariners , who had been harassed by Tokhta , to settle in Crimea . But the Mongols sacked Sudak under Khan Öz Beg in 1322 as a result of a clash between Christians and Muslims in the city . The Genoese merchants in the other towns were not molested in 1322 . The Pope intervened and asked Öz Beg to restore the Roman Catholic churches that were destroyed . Öz Beg was friendly towards the Pope and exchanged letters and gifts . Khan Öz Beg signed a new trade treaty with the Genoese in 1339 and allowed them to rebuild the walls of Kaffa . In 1332 he had allowed the Venetians to establish a colony at Tanais on the Don . New Sarai . During the reign of Öz Beg , Sarai ( literally meaning " palace " in Persian and Turkish ) was more quickly becoming a main commercial center and industrial trading center of the country rather than just a political center . The expression of Mongol camp mentality , following Ash and the nearby absence of some structures . To successfully spread Islam , it was necessary to build a mosque and other " elaborate places " requiring baths , an important element of Muslim culture . Sarai attracted merchants from European , Asian and Islamic countries as well as Middle East . Slave trade flourished due to strengthening ties with Mamluk Sultanate . Successful commercial revolutions require new markets , caravans : " places where merchants find their way . " Growth of wealth and increasing needs of production always produce population growth . Unfortunately this did not passover in Sarai . The increase of the regions dwelling places transformed the capital into a center of a large Muslim government , giving it the appropriate aspect and status . Öz Beg actually came to build a new city , which received the official name Saray al - Jedid or New Sarai . Relationship with Russian princes . Öz Beg supported the earliest princes of Muscovy - his brother - in - law Yury of Moscow ( or Yuri ) and Yury ' s successor Ivan Kalita ( or Ivan Daniilovich , later , Ivan I of Moscow ) - against their relatives , the westward - leaning Princes of Tver . Four of these latter rulers - Mikhail of Tver , his sons Dmitry ( or Dmitri ; nicknamed The Terrible Eyes ) and Alexander Mikhailovich , ( or Aleksandr ) and his grandson Theodor ( or Fyodor ) - were all killed in Sarai at Öz Beg ' s behest . In 1317 , Mikhail Yaroslavich defeated Yuri at a village called Bortenevo . Mikhail captured Yuri ' s wife , Konchaka , who was the Khan ' s sister . Unluckily , Konchaka died when she was in the custody of Mikhail ; Yuri announced to the Khan that she had been poisoned by order of Mikhail . He and Yuri were summoned to the court of Golden Horde for a trial , after which Mikhail was beheaded in December of 1318 . Following Yury ' s machinations , which prompted the Khan to grant the yarlik ( patent of office for the title of Grand Prince of Vladimir ) to Moscow and led to their father ' s execution by the Horde , Dmitry and his brother , Alexander , fought a series of battles with Yury and intrigued against him at the Horde , culminating in Dmitry ' s acquisition of the yarlik of office for the grand princely throne in 1322 , after he had persuaded the khan that Yury had appropriated a large portion of the tribute due to the Horde . Yury was summoned to the Horde in Sarai for a trial but , before any formal investigation , was killed by Dmitry , November 21 , 1325 . Öz Beg waited to punish Dmitri and eventually he arrested the Prince of Tver for the murder , executing him in 1326 . When the Khan ' s cousin , the Baskaki Shevkal , and his Tatars were killed in Tver and a rebellion erupted there in mid - August 1327 , Dmitry ' s successor as Prince of Tver and Grand Prince of Vladimir - Suzdal , his brother Alexander Mikhailovich , fled first to Novgorod - - - which turned him away - - - and then to Pskov - - - which made him it ' s Prince - - - in order to escape a punitive expedition of 50 , 000 Mongol - Tatars & Muscovites , which was headed by his cousin , Yury ' s brother & successor Ivan Daniilovich . Tver ' s uprising against the Horde was bloodily suppressed by the Muscovite and Tatar forces in 1327 . Öz Beg appointed Ivan to position of Grand Duke of Vladimir in 1328 ; that marked the true beginning of the rise of the Muscovites . Alexander , after many travails , including exile in Sweden & Lithuania , eventually prospered in Pskov , under the patronage of Gediminas , Grand Duke of Lithuania . He sent his son Fyodor to the Horde , with tribute and seeking forgiveness , in 1335 ; the Khan pardoned Alexander and he was given the princely yarlik to Tver once again in 1337 . Unfortunately , his greatest enemy , his cousin Ivan , once more set the Horde ' s Khan against him with the aid of intrigue . Alexander was summoned to the Horde again and was executed at the hand of Khan Öz Beg ; Tver was then pillaged and many of its citizens massacred . Öz Beg welcomed Ivan ' s sons and made Simeon Grand Prince ( duke ) in 1340 . Simeon was given more powers by the Khan to counter Lithuania ' s growing power . Öz Beg also launched military expeditions into Lithuania , as it was clear that it threatened Mongol dominance in Russia . Relationship with Ruthenian Princes . After Öz Beg ' s army killed Lev II and his brother Andrey ( co - kings of Galicia - Volhynia , and last of the Rurikid Dynasty ) in 1323 , the Grand Duchy of Lithuania and the Kingdom of Poland had great influence over Galicia - Volhynia . The Lithuanians defeated the Rus ' boyars and occupied Kiev and its surrounding areas in 1330 . As a result of Öz Beg losing direct rule over Kiev , Wallachia and its ruler Basarab I became de facto independent after about 1324 . But Öz Beg was still able to threaten Byzantium , Lithuania and Bulgaria .