Complex sentence: Owain was a soldier who served in Spain, France, Alsace and Switzerland; in French service he had good relations with Bertrand du Guesclin and others and gained the support of Charles V of France.
Split into simpler sentences: Owain was a soldier who served in Spain, France, Alsace and Switzerland. While in French service he had good relations with Bertrand du Guesclin and others and gained the support of Charles V of France.