PoD: The Fourth Crusade goes even worse than OTL, allowing Constantinople to fall to Muslim invaders months afterwards.

After the fall of Constantinople to the Crusaders, the unstable Latin Empire was founded as a last resort. However, it quickly fell to the hungry Rums, who killed the Emperor to assure their succession to the Latin throne. After the fall of Constantinople in the 1300’s, they quickly advanced to take over Greece, and then moved north to fight the Golden Horde, Bulgaria, and Austria. Having centralized their power beyond that of the smaller countries, the Rums managed to quickly expand into Central Europe. In 1485, Austria and its allies of Bavaria and Venice fell to the Muslim alliance. The Pope called for another crusade, but it was much too late. The armies of many Papal allies had been turned against them, and the subjugation of Christianity was at hand. The Spanish quickly focused on the Reconquista, making sure the Rums wouldn’t take any of their precious Iberian Peninsula. It was reminiscent of the eighth century, and essentially moved Europe into a second Dark Age. The Rums and its new puppets of Venice and Sicily fought a valiant war against the Papal States, a war which they eventually won in 1502.

A while after the conquest of the Christian church, a French explorer stumbled upon the continent of Alpin-rouge (OTL Central America/Caribbean). As the French were at war with the Muslims at the time (Eventually ending with the conquest of Avignon and most of Occitania), a mass exile to the former Aztec empire ensued. From 1560 to 1600, a large influx of European settlers from all walks of life moved into the region, many desperately getting jobs to convert the populous. The three allied cities of Tenochtitlan were taken over by a French-English-Italian alliance, with soldiers from Moravia and even Poland assisting with the conquest. With their homelands gone, the Europeans united together to form a new empire, which they named Zephyrus (After the Roman name for the southeastern wind). The church in exile founded new cities to base their Christian ideals off of, while much of Europe moved from their home continent to Alpin-rouge. The Spanish were tasked with taking down the Inca and the Mapuche, while the Zephyrussians worked to take down the Aztecs. The Rums eventually colonized Mustarshida, their name for OTL North America east of the Mississippi.

However, there was one group who was not too happy with this exodus; the natives. While much of the former Aztec empire was converted to Christianity, the believers in the Aztec religion still existed on the fringes. In a strange showing of unity between the Aztec people, a crusade was called against the Christians. They quickly mobilized an Aztec army to take down Tenochtitlan, and assassinated the Zephyrussian emperor, along with a few high-ranked Christian political figures. Soon enough, the empire was fractured, dividing into two factions. There was the official continuation of Zephyrus, belonging to the French settlers in Tlacocum (the Latin name for the area around Tenochtitlan) and New Carcassonne, and the “Union of the Church” in Avignon and New Latium. The Aztec crusader states, founded in majorly Christian regions, got along okay with the “Restored Aztec Empire”, the new major power in the area. The states remained fractured for decades, some dominating over others. By 1675, the major powers were the Restored Aztec Empire, the Tlacocum Remnant of Zephyrus, and the majorly-French (but Aztec-run) Yancuictlan. Eventually, the Aztecs would be pushed out for good, but they had the upper hand for a long while.