Leon C. Thrasher probably never would have acquired historical fame had he not booked passage aboard the steamer Falaba in early 1915. The ship was torpedoed on 28 March, and he died, the first American victim of German unrestricted submarine warfare.
The event stimulated a great debate between State Department counsellor Robert Lansing and the Secretary of State William Jennings Bryan. Lansing followed the view that Germany had violated traditional methods of warfare resulting in the avoidable death of an innocent neutral, while Bryan argued that Thrasher had been negligent in choosing a British vessel sailing to a designated war zone where it was at risk of being attacked and sunk. Wilson took his time in deciding between these two views of the incident, in part because Colonel Edward House, his personal envoy, was in Europe seeking to attract the warring nations to the idea of a mediated peace.
While President Woodrow Wilson pondered his response, the Germans made two further attacks that challenged neutral rights in the war zone. An American freighter, the Cushing, was attacked by a German aircraft on April 29, while on 1 May the tanker Gulflight was torpedoed by the U-30. At the following cabinet meeting, on 4 May, Bryan found the mood of the cabinet was for a protest. Wilson stated: "It may be that there is no way to meet a situation like this except by war. It is important that we should show how sincere is our belief that there are other ways to settle questions like this." The tension in the Democratic party and the Progressive movement could not be made any more clear. To threaten war was the traditional response, but the Democrats had hoped to avoid "bad" European traditions in the creation of new, "good" American ones. However, there was no guarantee that the rest of the world would go along.
The sinking of the Lusitania on 7 May 1915 was the turning point for American diplomacy in the war, although it was not apparent in the days immediately afterwards. That the Lusitania was carrying ammunition, a legitimate target as contraband of war, is not disputed. However, sinking a passenger liner without warning, which is what U-20 did, was at the time an action that was open to question as to its legitimacy. Therefore the Wilson administration's response was bound to be stern.
[NB - This strand has become longer and longer as I study more about it, and it might be argued I have lost my way somewhat. However, I think the extensive background will help when I come to the anti-war movement.]