In school, we are currently researching the Atlantic Slave Trade, and today we read two letters from King Alfonso I/Nzinga Mbemba of Kongo to King João III of Portugal. I researched a bit more about the Kingdom of Kongo and its history, and I found it really interesting how the Kongolese emperors adopted Christianity and various Portuguese customs in their empire without being conquered until the early 20th century.
So, that brings me to my question; what would it take for a large empire in the New World like the Aztecs or Inca (Aztecs just came to mind first for the title) to convert to Christianity and adopt various European customs without being conquered (for a while at least)?