Complex sentence: The course of the war saw extensive fighting in Italy, France, and the Low Countries, as well as attempted invasions of Spain and England; but, although the conflict was ruinously expensive for the major participants, its outcome was inconclusive.
Split into simpler sentences: The course of the war saw extensive fighting in Italy, France, and the Low Countries, as well as attempted invasions of Spain and England. The conflict was ruinously expensive for the major participants, and its outcome was inconclusive.