You can credit most of it to Altan Khan who had close ties to the Yellow Hat Sect of Tibet (Altan actually coined the title "Dalai Lama") and brought it into Mongolia in a big way starting in the 16th century. That's really where modern Mongolian Buddhism started. Here's Altan Khan's wiki for more details: en.wikipedia.org/wiki/Altan_Khan
Of course the Mongols had brushes with Buddhism before that, in the time of the Yuan dynasty. Khubilai Khan was very supportive of Buddhism. And before that the Khitans also embraced Buddhism (especially Tantric Buddhism) and spread it to some other steppe people like the Uighurs and the Naimans (the last Naiman chief Kuchlug was a Buddhist).
And before that the Khitans also embraced Buddhism (especially Tantric Buddhism) and spread it to some other steppe people like the Uighurs and the Naimans (the last Naiman chief Kuchlug was a Buddhist).
Actually the Uyghurs were Buddhist as early as the 9th-10th centuries, before the Qara Kitan was founded. And Küchlüg, originally a Nestorian Christian, accepted Buddhism just to marry the daughter of the Gür Khan, right?
Nomad, rider of the ancient east
Nomad, rider that men know the least
Nomad, where you come from no one knows
Nomad, where you go to no one tells
Though Buddhism was already starting to spread in Mongolia the Manchus did much to spread it, thinking it would make the Mongols "behave better" and be less warlike, at least thats the claim in Mote's Late Imperial China book.
“With Heaven's aid I have conquered for you a huge empire. But my life was too short to achieve the conquest of the world. That task is left for you.” ~Chinggis Khaan
In fact, the spread of Buddhism in Mongolia (if we talk about the territory of modern Mongolia) during Liao and Yuan dynasty was limited. Mongols still mostly kept their traditional Tingreism; also Nestorian Christianity was practiced by some tribes. The Tibetan style Buddhism had started to spread in Mongolia in the 16th century before Manchus and was already quite firmly established before the complete Manchu conquest.
usually steppe people have been more inclined to 'smaller' religions, like the Khazar nobility adopting judaism even though bordering the caliphate as well as christian byzantium. same goes for the steppe around the time of temujin, nestorian christianity was popular even though there was no nearby state (or any state) to back or spread it. so i don't see why buddhism should have had any impact on the steppe, there's also no evidence for it in the secret history of mongols.
Prior to Chinggis Khaan, the area that is Modern Mongolia was mostly controlled by Turkish speaking tribes, including the Uighers.
I pointed out that to the south of Mongolia the Xia kingdom (1038 - 1227) was a Buddhist state, Also the Liao Dynasty (Khitan, 907 - 1125) which had fortified postitions throughout the Area of Modern Mongolia, also practiced Buddhism.
Added to that, in the book "The Great State of White and High" is mentioned Uigher Buddist Monks traveling around. It would have been probable that they would have traveled into the areas of Mongolia inhabited by Turkish speaking Tribes. However, at this time that is conjecture.
We do know that Hohhot has a long history of Buddhism and was frequently visited by Mongols from the areas of Modern Mongolia.
So it is probable that there was some Buddhism in the Area of Modern Modern Mongolia around 1000 AD.
It is clear from the Secret History, that Chinggis Khaan was not a Buddhist. However, Khubilai Khaan certainly supported Buddhism and it is known that at KharaKorum there were Buddhist temples.
So by 1240s, Buddhist is known among the Mongols.
In my previous note, I mentioned the Buddhist documents from the 14th - 15th centuries that were found in Mongolia.
In the History of Mongolia by H.H. Howorth, he mentions Buddhism often. In particular, he mentions documents in Russia found by Johann Eberhart Fischer that document some Russian envoys trips to a Khan of the Western Khalkas named Ombo Erdeni and later his son Shului Ubashi. Their camp was at Uvs Lake in North Eastern Mongolia. These trips began in 1616. See page 459 - 472. the envoys mention Buddhist Lamas who were advisors to the Khans and that the Khan and his people were Buddhist.
So we see that by 1616 Buddhism had spread to the Northern Borders of Mongolia. Uvs Lake (N50 18 45.70, E92 43 18.12)
Thus Buddhism seems to be widely established before the Manchus Qing Dynasty appears.
Of course, Buddhism established itself by the 17th century, but that was the Tibetian version of Buddhism. Buddhism practiced by Liao dynasty and also Xia state was just a variation of traditional Chinese Chan Buddhism. There are no traces of that left in Mongolian culture. The Tibetian Yellow faith really started to establish itself in Mongolia in the 16th century.
I think I've read that Tibetan Sakya tradition of Buddhism was popular amongst the Mongols (and especially in Buryatia) before etablishig Gelug (a.k.a. the Yellow faith) as official, but I may be wrong about that. At any case it was Sakya school that was patronized by Khubilai Khaan.
"The Gelug or Gelug-pa (or dGe Lugs Pa, dge-lugs-pa, or Dgelugspa), also known as the Yellow Hat sect, is a school of Buddhism founded by Je Tsongkhapa (1357–1419), a philosopher and Tibetan religious leader. The first monastery he established was at Ganden, and to this day the Ganden Tripa is the nominal head of the school, though its most influential figure is the Dalai Lama. Allying themselves with the Mongols as a powerful patron, the Gelug emerged as the pre-eminent Buddhist school in Tibet since the end of the 16th century."
So clearly, any Buddhism prior to 1400 was not this.
Under Buddhist timelines in Wikipedia is found:
"1295: Mongol leader Ghazan Khan is converted to Islam, ending a line of Tantric Buddhist leaders."
About the Xia state( from "The Great State of White and High")
1. A type of Tantric Buddhism was practiced.
2. Had Buddhist monks from India and also some Uigher monks translating Buddhist material.
3. Ethnically close to Tibetans, speaking a Tibetan dialect.
At this period 1000 - 1200
The three states (Xia, Song and Liao) were competing and fighting. They also were exchanging Buddhist material. I would say that all contributed to the later development of Buddhism in North China and Mongolia.
And yes, the GELUK tradition of Buddhism became predominate in Mongolia. If you read "The History of the Mongols" by H.H. Howorth, he talks about some of the history of these conversions to the GELUK school.