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