In August 1914 the Germans invaded Belgium giving their submarines direct access to the North Sea via Zeebrugge.
They built heavily defended submarine pens inland at Bruges, out of range of the guns of the Royal Navy's battleships
The submarines travelled to the coast via the Zeebrugge and Ostende canals and on into the North Sea, the English Channel and the Atlantic
German submarines sank a huge amount of merchant shipping in an attempt to starve Britain out of the War
Zeebrugge was a vital part of this strategy and was very heavily defended and German naval vessels were based on the Mole
And submarine nets were laid across the harbour entrance