Complex sentence: The marriage was arranged by King Gustav III to provide the throne of Sweden with an heir; the king had not consummated his marriage and had decided to give the task of providing an heir to the throne to his brother.
Split into simpler sentences: The marriage was arranged by King Gustav III to provide the throne of Sweden with an heir. The king had not consummated his marriage and had decided to give the task of providing an heir to the throne to his brother.