However, the German general staff did not agree. In April 1916, Casement returned to Ireland in a U-boat and was captured and executed. A German ship (the "Libau") renamed the "Aud", flying Norwegian colours, shipped 20,000 rifles to the south Irish coast, but it failed to link up with the rebels and was scuttled. Planning on this support, the Irish Volunteers launched the Easter Rising in Dublin. Though the Rising failed, its political effect led on to the Irish War of Independence in 1919–22 and the formation of the Irish Free State. On 6 August 1917, Zimmermann resigned as foreign secretary and was succeeded by Richard von Kühlmann. One of the causes of his resignation was the famous Zimmermann Telegram he sent on 16 January 1917. He died in Berlin in 1940 of pneumonia. Two and a half years into World War I, the United States had maintained a status of neutrality while the Allied armies had been fighting those of the Central Powers in the trenches of northern France and Belgium. Although President Woodrow Wilson had been re-elected – winning the election on the slogan, "He kept us out of the war" – it became increasingly difficult to maintain that position. After the Royal Navy had been engaged in a successful naval blockade against all German shipping for some time, the German Supreme High Command concluded that only a total submarine offensive would break the stranglehold. Although the decision was made on 9 January 1917, the Americans were uninformed of the operation until 31 January. The Germans abrogated their Sussex pledge (not to sink merchant ships without due warning and to save human lives wherever possible) and began an unrestricted U-boat campaign on 1 February 1917. Since it was obvious that US shipping would also come under attack in the course of this operation, it became just a matter of time before the USA was drawn into the conflict. Germany had been pursuing various interests in Mexico since the beginning of the 20th century. Although a latecomer in the area, with Spain, Britain, and France having established themselves there centuries earlier, the Kaiser's Germany attempted to secure a continuing presence. This entailed many different approaches to the Mexican Republic and its changing, often revolutionary, governments as well as assuring the United States (most of the time) of Germany's peaceful intentions. German diplomacy in the area depended on sympathetic relations with the Mexican government of the day. Among the options discussed during Arthur Zimmermann's period in office was a German offer to improve communications between the two nations and a suggestion that Mexico purchase German submarines for its navy.