Complex sentence: The British saw the German use of Ostend and Zeebrugge as threats to their naval supremacy in European waters and that reinforcement shipping from Britain to France could be threatened by German submarines and ships.
Split into simpler sentences: The British saw the German use of Ostend and Zeebrugge as threats to their naval supremacy in European waters. Should this materialise, reinforcement shipping from Britain to France could be threatened by German submarines and ships.