Unrestricted submarine warfare is the practice of using submarines to attack and sink all forms of enemy shipping, whether they are military or civilian. It is most closely associated with the First World War when Germany's decision to use USW brought the US into the war and led to their defeat.
The Blockades of World War 1
In the build-up to the First World War, Germany and Britain were involved in a naval race to see how many bigger and better battleships could be created. When this war began, many expected the resulting navies to sail out and fight a great naval battle. In fact, this only ever almost happened at Jutland, and that was inconclusive. The British knew that their navy was the only part of their military who could lose the war in an afternoon and decided not to use it in a massive battle but to blockade all the shipping routes to Germany and try and starve their enemy into submission. To do so they seized the shipping of neutral countries and caused a lot of upset, but Britain was able to soothe ruffled feathers and come to agreements with these neutral countries. Of course, Britain had the advantage, as it was between Germany and the Atlantic shipping routes, so US purchases were effectively cut off.
