Complex sentence: The aim of the Scots was simple enough: to ensure that the Philip acquired a direct interest in both the survival of the nation and its monarchy; but in signing the alliance they had also effectively signed a declaration of war against England.
Split into simpler sentences: The aim of the Scots was simple enough: to ensure that Philip acquired a direct interest in both the survival of the nation and its monarchy. However, in signing the alliance they had also effectively signed a declaration of war against England.