The German defensive strategy on the Western Front in 1917, succeeded in resisting the increase in the offensive power of the Entente, without the loss of vital territory but the attrition of German manpower was slowed rather than reversed. Unrestricted submarine warfare caused the United States to declare war on 6 April and failed to isolate Britain from its overseas sources of supply. The bombing offensive against Britain, acted to divert Anglo-French air defence resources, which slowed the rate at which the German air service was outnumbered in France. By the end of the Third Battle of Ypres in November 1917, the effectiveness of the methods of defence introduced in 1917 had been eroded and continuation of a defensive strategy in the west was made impossible. The defeat of Russia gave the German leadership a final opportunity to avoid defeat, rather than the attempts to compete with Allied numerical and industrial superiority, through economic warfare in the Atlantic and the domestic initiatives of the Hindenburg Programme, the Auxiliary Service Law and temporary demobilisation of skilled workers from the army.