Complex sentence: Philip II of Spain disliked the idea of assassinating Elizabeth; a stable England was needed as a counterweight to France; there was no guarantee that the English population or its nobility were as Catholic in sentiment as the success of the plot demanded.
Split into simpler sentences: Philip II of Spain disliked the idea of assassinating Queen Elizabeth, as a stable England was needed as a counterweight to France. There was also no guarantee that the English population or its nobility were as Catholic in sentiment as the success of the plot demanded.