One of the great lessons of the Great War was the inter-relation of international politics and warfare. It was an old lesson indeed, but modern conditions emphasized it once more. We have already seen that the torpedoing in 1915 of the Atlantic liners _Lusitania_ and _Arabic_ caused pressure to be put on the German Government by the United States of America. In the spring of 1916 the submarine campaign, for the Germans, was proceeding very satisfactorily. In February they had sunk 24,059 tons of British merchant shipping, in March they sank 83,492 tons, in April 120,540 tons; but in May this dropped suddenly to 42,165 tons. What was the reason for this sudden fall? The answer is as follows: On March 24, 1916, the cross-Channel S.S. _Sussex_ was torpedoed by a German submarine, and it happened that many citizens of the U.S.A. were on board at the time and several were killed. This again raised the question of relations between the U.S.A. and Germany, the _New York World_ going so far as to ask, ‘Whether anything is to be gained by maintaining any longer the ghastly pretence of friendly diplomatic correspondence with a Power notoriously lacking in truth and honour.’ On April 20, therefore, the U.S.A. presented a very sharp note to the German Government, protesting against the wrongfulness of the submarine campaign waged versus commerce, and threatened to break off diplomatic relations. The result of this was that Germany had to give way, and sent orders to her naval staff to the effect that submarine warfare henceforth was to be carried on in accordance with Prize Law: that is to say, the U-boats—so Admiral Scheer interpreted it—were ‘to rise to the surface and stop ships, examine papers, and all passengers and crew to leave the ship before sinking her.’ Now this did not appeal to the German mind at all. ‘As war waged according to Prize Law by U-boats,’ wrote Admiral Scheer,[3] ‘in the waters around England could not possibly have any success, but, on the contrary, must expose the boats to the greatest dangers, I recalled all the U-boats by wireless, and announced that the U-boat campaign against British commerce had ceased.’ Thus we find that after April 26 the sinkings of British merchant ships became low until they began to increase in September, 1916, and then rapidly mounted up until in April, 1917, they had reached their maximum for the whole war with 516,394 tons. It is to be noted that after May 8, until July 5, 1916, no sinkings by U-boats occurred in home waters, although the sinkings went on in the Mediterranean, where risk of collision with American interests was less likely to occur.