Is the following alternate history setup conceivable:
- Central Mexico has been converted to Buddhism by missionaries arriving from East Asia or South-East Asia hundreds of years before European conquistadors arrive.
- By the time of European conquest of the Caribian, Central Mexico is organized as a centralized and predominantly Buddhist state.
- The missionaries brought scriptures (naturally) and some knowledge of East Asian technology (advanced metal working, steel, the wheel, siege weapons, etc.).
- There is no continued political interference from Asia in the Americas, certainly no military interference.
- With the Pre-Columbian contact, Native Americans have acquired some immunity against Eurasian diseases.
Note that with a very different timeline, the people in Central Mexico would not be called Aztecs; I am just calling them that for want of a better universally understandable term. (In fact, they were not called Aztecs; this term is an invention of 19th century scholars.)
What it could look like
On Holy Thursday, the anniversary of the Last Supper of Our Lord, in the year of Our Lord 1519, the Captain-General ordered us and all his fleet to disembark in this strange new land. While we made camp between the dunes we noticed a shrine with a heathen idol nearby, a gilded statue showing a monstrous sitting figure with six arms and a serene if gloating smile. The Captain-General ordered the false idol removed and replaced by the likeness of Our Lady.
For some days, the Indians native to this country avoided us, until an embassy arrived on Easter Sunday. The embassy was led by an Indian of noble birth, Teuhtlilli, Governor of Cuetlaxtla, accompanied by many servants and warriors. Teuhtlilli brought presents of fowl and vegetables and greetings of the prince of this country, who was called Ahuitzotl the Enlightened. Among Teuhtlilli's people, there were several barefooted men with stoic expressions clad in nothing but an orange robe. We learned that they were heathen monks, learned in religion and all the arts and the Indians greatly valued their council. The warriors, on the other hand appeared in full metal armor, with lances and steel swords and steel helmets adorned with golden images of snakes and dragons and terrible monsters. They assured us that there were indeed fire-breathing dragons that inhabited the mountains of the lands ahead.
The Captain-General ordered an altar made. Fray Bartolome de Olmedo celebrated the Holy Mass in the presence of the Captain-General and his men and the Indian embassy, for the Captain-General wanted to show the heathens the way of the true religion. Afterwards, the Captain-General dined with the Indian ambassador. He also arranged a demonstration of the cannon so that the ambassador should see it fired. The ambassador was thoroughly unimpressed. He seemed insulted and demanded to know why we had defiled the shrine to the Bodhisattva Quetzalcoatl. The Captain-General tried to explain our holy faith, but Teuhtlilli would not hear it. He demanded that we re-embark and leave this country immediately; then he departed abruptly.
The next day, we learned that an Indian army had arrived and was encamped not far from where we had made landfall. They numbered in the tens of thousands, all with formidable armor and weapons made of steel. They also brought a number of trebuchets; one of our ships that came too close to the Indian camp was hit by a flaming projectile and set ablaze...
(Note that style and some of the wording and context is adapted from Bernal Diaz' account of Hernan Cortes' conquest of Mexico.)
How it could have happened
For a time, Buddhism was aggressively prosetylizing everywhere from Japan, to Central Asia, to modern Indonesia, to Egypt and Greece. An apparent Buddhist missionary who came with an Indian embassy burned himself to death publicly in Athens in 22/21 BCE to demonstrate his faith. Buddhist missionaries may have been government-sponsored but were happy to work alone, under hardship, and far away from their countries of origin. They would certainly have been willing to go to Mexico, had they known how.
There was speculation about whether Buddhist missionaries may actually have reached the Americas in Pre-Columbian times. Around 500 CE, a group of Buddhist monks traveled from China to a country called Fusang, across the sea 20000 li East of China. One li is between 300 and 600 meters (depending on where and when), which is not quite far enough to be America but much further away than Japan. Traditionally, it has nevertheless been interpreted to refer to some part of Japan or Sakhalin or other islands.
Further, in later times, it happened frequently that Japanese fishing boats were driven out into the ocean and across the Pacific. Some Japanese sailors survived and were taken captive by Native Americans in the 19th century. It seems conceivable that this might have happened in a similar way 1000 years earlier to Buddhist missionaries bound for Japan, China, or Korea.
- While Japanese fishermen carry potentially enough food (or at least the tools to acquire some) for the long journey across the Pacific, a ship sailing the rather short way between Korea and Japan, or even between South-East Asia and Japan would probably not.
- Buddhist missionaries in other regions were most successful when relying on a network for ideological (perhaps also material) support. Buddhist states and Buddhist monasteries could provide that across Asia, but not in the Americas. Especially when considering that the missionaries would not have had any way of knowing where they are, how to get home or write home and report about their success, etc. Even with nautical knowledge, traveling reliably across the Pacific (in order to get back, even if you understand where "back" is), requires understanding the volta do mar. The Spanish had to try for a while and lost many ships before they got it right.
- The religious establishment in Central Mexico would not be amused.
- Technological knowledge is difficult to transfer. The Mongols for instance had to bring in Persian engineers to build counterweight trebuchets and reduce the Song fortresses. Buddhist missionaries may not be concerned with either steel or siege warfare or anything of the sort. Even if they had seen this in Asia and were willing to help by reproducing this technology in Mexico, they may not succeed. Or possibly they would produce very crude versions of it.
- The wheel may not be useful enough without large draft animals. But missionaries would not accidentally bring horses or water buffalo (or perhaps they would eat those when starving at sea).
- It may be impossible to move a trebuchet without draft animals.
- A one-time or two-time contact with a few missionaries may not have been enough to spread Eurasian diseases to the Americas and cause immunity to develop.
- Iron ore may not be easily to find in or around Central Mexico.