In 1917, Britain was just six weeks away from starving to death.
We have the same population as France, but only half the landmass.
So we can’t grow enough to feed ourselves.
Which means most of what we eat must be imported.
And Germany had worked out that the best way to win World War One was to cut Britain’s jugular.
The constant supply of food from the USA and Canada.
And the most efficient way to do that was with U boats.