Complex sentence: Despite this there were ominous signs for the Spanish, as the combined French and Spanish fleets had been unable to gain mastery of the seas and had also failed in two of their key objectives, regaining Gibraltar and an invasion of Great Britain.
Split into simpler sentences: Despite this there were ominous signs for the Spanish, the combined French and Spanish fleets had been unable to gain mastery of the seas. They had also failed in two of their key objectives, regaining Gibraltar and an invasion of Great Britain.