Complex sentence: In the next 25 years, 15 missions were founded in the province of Guayrá--but since some of these were within the Portuguese area they were subjected to frequent destructive raids by Bandeirantes of São Paulo to enslave the Indians.
Split into simpler sentences: In the next 25 years, 15 missions were founded in the province of Guayrá. But since some of these were within the Portuguese area they were subjected to frequent destructive raids by Bandeirantes of São Paulo to enslave the Indians.