(A) the Sussex Pledge
(B) the interception of the Zimmerman telegram
(C) the sinking of the Lusitania
(D) ** unrestricted submarine warfare
EXPLANATIONS BELOW
Concept note-1: -Blockades, part of economic warfare, had been employed throughout history. The Allied blockade (1914-1919), which aimed to prevent war supplies reaching Germany, ultimately also targeted the civilian population. It had its counterpart in the German attempt to block supplies to the Entente.
Concept note-2: -One U-boat would shadow a convoy and summon others by radio, and then the group would attack, generally on the surface at night. These tactics succeeded until radar came to the aid of the escorts and until convoys could be given continuous sea and air escort all the way across the Atlantic in both directions.
Concept note-3: -Unrestricted submarine warfare was first introduced in World War I in early 1915, when Germany declared the area around the British Isles a war zone, in which all merchant ships, including those from neutral countries, would be attacked by the German navy.
Concept note-4: -Germany’s resumption of submarine attacks on passenger and merchant ships in 1917 was the primary motivation behind Wilson’s decision to lead the United States into World War I.
Concept note-5: -The army would attack Hitler’s troops at their weakest points first and slowly advance toward German soil. The plan was known as “closing the ring.” In December 1941, Roosevelt and Churchill agreed to attack German holdings in North Africa first. That maneuver was finally executed in October 1942.