Yes!
Brazilian here, they were called the Society of Jesus, a catholic order founded by Saint Ignatius of Loyola and worked mainly on Iberic nations and it's territories, like Brazil.

The society aim was to convert native population to catholicism, in agreement with the Counter-Reformation. So, they were strictly against indigenous enslavement, for they wanted the natives to become new converts, while the colonists needed slaves for the sugar plantations (and later the gold mines).

Unfortunately, the society believed the best way for conversation was to "reduct natives into civilization", building "Reductions", basically a village formed by the priests and the natives. This was catastrophic, in the northeast it resulted in a lot of deaths caused by infectious diseases and colonist's attacks.

Many Reductions were build in what is now south of Brazil, but at the time, according to the Tordesilhas treaty, was Spain. This reductions suffered decades of incursions by the portuguese colonists in São Paulo, which was a poor and distant city of 1700' Brazil (the economy was mainly based in sugar (Northeast region untill mid century) and gold (Minas Gerais region after mid century) and had an economy based on native slave trade.

These incursions by the so called "Bandeirantes" pushed the Reductions south, untill they were "safely" established in nowadays Rio Grande do Sul, a brazilian state.

What you may be refering is called the "Guerra Guaranítica", i think that in english is something like "Guaranitic War", it happened around 1750' and happened on said territory. After many wars between Spain and Portugal (and it's colonies in Argentina and Rio Grande), a new treaty was signed, the treaty of Madrid, which gave Sacramento (a portuguese colony right crossing the Prata river in nowadays Uruguai) to Spain in exchange of some territories east of the Uruguai river to Portugal.

The thing is, by this time, the Society of Jesus was banned in Portugal because of the current regent, Marquês de Pombal, policies, but they still were legal in Spain. So, when they were granted this new land, they tried to expel the priests established there for more than half a century, but they deny it.

The seven missions in the region, led by Sepé Tiaraju and the monks, fought against the colonial armies of both Spain and Portugal, being ultimately defeated and enslaved. There is a very famous book (writed as a Marquês de Pombal propaganda) called "The Uraguai".

The Society of Jesus is a fascinating organization, it had a lot "saints", teachers, translators and influence. In colonial Brazil they were basically in charge of education and conversion, the famous writer "José de Anchieta" was a jesuit and you can still find some jesuitic schools around Brazil.

Sorry about the lenght, i really like the subject hahaha feel free to ask me about it