Maynas was one of the missions the Jesuits created in South America. The missionaries started their descent from Quito in Ecuador mostly using the Rio Napo as their route. They used the common missionary techniques, found in other Jesuit missions like Paraguay, Chiquitania, Moxos or Orinoco. A difference from other areas was that the Indians belonged to different tribes and languages.
The missions expanded under Samuel Fritz SJ (who traveled all the way to Belém and drew the first map of the Amazon) up to what is today Manaos. This provoked trouble with Portuguese slave traders, who travelled from Belem upstream. It never was as stable or prosperous as the missions in Paraguay, but as one result Portuguese influence was stopped, and the upper parts of the Amazon basin fell to Spanish-speaking countries.
After the expulsion of the Jesuits, Maynas came under the control of Franciscans from Oacampa-Peru. This was one of the reasons the border and the ownership of Maynas in the post-colonial time was unclear. It provoked several wars between the two countries, before 1996 (Cenepa).