Complex sentence: Grenville was denied a command under Drake in the successful raid on Cadiz in 1587, and contented himself with organising the defences of Devon and Cornwall in preparation for the arrival of the Spanish Armada the following year.
Split into simpler sentences: Grenville was denied a command under Drake in the successful raid on Cadiz in 1587. He organized the defences of Devon and Cornwall in preparation for the expected attack by the Spanish Armada the following year.