What did the Germans pledge they would do to any Allied vessel sailing around Britain?

Answer: provide warning before shooting with u-boats and then shoot it down

The correct answer is provide warning before shooting

Germany had the largest submarine fleet among the belligerent countries. However, the commanders of these vessels had been refraining from torpedoing passenger ships (albeit with an enemy flag) and any ships from neutral countries. The exception was the English transatlantic Lusitania, torpedoed in 1915 and which exploded - probably because it was secretly transporting American ammunition to England.

In January 1917, the German government announced that it would launch an "unrestricted" underwater campaign; that is, their submarines would torpedo any ships trying to reach French or British ports. This decision complicated the situation of the Allies, since Great Britain depended on sea supplies for its own survival.

In March 1917, the Russian Revolution broke out. Tsar Nicholas II was overthrown and a liberal provisional government (formed by aristocrats and bourgeois) took over. Officially, Russia continued in the war against Germany; but his soldiers, exhausted and demoralized, practically stopped fighting. This circumstance could allow the Germans to deploy troops to the western front, definitively defeating the English and the French.

In the course of the war, the United States had become a major supplier to the Allies, to which it sold everything from food to weapons and ammunition. Great Britain, France and other countries had accumulated enormous debts with American businessmen, who could not bear the fantastic loss resulting from a possible Anglo-French defeat.