The Royal Navy stops ships carrying war supplies and food from reaching Germany.
As soon as war was declared Britain's Royal Navy began a blockade of Germany. Any ships heading for Germany had to be inspected and anything considered contraband (not allowed) was kept back. This contraband included food and by the end of the war the people of Germany were starving as a result of the blockade. Denying the German people food broke international law and other countries including the United States were angry about it at first.
The British Royal Navy was the most powerful naval fleet in the world. Britain used it to blockade Germany.