Agreement by Germany to stop using unrestricted submarine warfare?

Answer 1

The sinkings of the RMS Lusitania and ferry Sussex caused enough public backlash against Germany that Germany changed its policy of unrestricted submarine warfare to a policy of attacking only ships that were clearly British. Neutral flagged and passenger ships would be afforded safe passage. Prior to this, U-boats operated under "Cruiser Rules", which effectively gave the ship warning, and time to evacuate before it was sunk. With the advent of air power and subsequent orders to ship captains to ram any surfaced U-boat, that policy became extremely hazardous.

The policy of restricted warfare lasted until 1917, when Germany, responding to the effective blockade of Germany by Britain, once again executed unrestricted submarine warfare against any vessel thought to be an enemy or providing assistance to the Allies. This decision ultimately led the U.S. to declare war on Germany, which followed with Germany's defeat in 1918.

It is notable to consider the possible outcome of the war if Germany had continued with unrestricted warfare rather than stopped. The Nazi U-boat campaign in the North Atlantic, as well as the American campaign in the Pacific, caused devastating losses and effect to both sides during WWII. The U-boats eventually were defeated in the Atlantic after the Allies developed sound anti-submarine warfare techniques, but it took time. The Japanese never countered the U.S. submarine threat in the Pacific, and it helped lead to their defeat. ASW techniques were essentially non-existent in WWI - had the Germans continued with their USW policy, the war may have gone much differently.