Unrestricted submarine warfare in the context of "United States declaration of war on Germany (1917)"

U-boats are naval submarines operated by Germany, especially during World War I and World War II. The term is an anglicized form of the German word U-Boot [ˈuːboːt] , a shortening of Unterseeboot (lit. 'under-sea boat'). Austro-Hungarian Navy submarines were also known as U-boats.

U-boats are especially known for their use in unrestricted submarine warfare in both world wars, attempting to disrupt merchant traffic towards the UK and force the UK out of the war. In World War I, Germany intermittently waged unrestricted submarine warfare against the UK: a first campaign in 1915 was abandoned after strong protests from the US but in February 1917 the Germans, facing deadlock on the continent, saw no other option than to resume the campaign. The renewed campaign failed to achieve its goal mainly because of the introduction of convoys. Instead the campaign ensured final defeat as the campaign was a contributing factor to the entry of the US in the First World War.

The Hindenburg Line (Siegfriedstellung, Siegfried Position) was a German defensive fortification built during the winter of 1916–1917 on the Western Front in France during the First World War. The line ran from Arras to Laffaux, near Soissons, on the Aisne. In 1916, the Battle of Verdun and the Battle of the Somme left the German western armies (Westheer) exhausted and on the Eastern Front, the Brusilov offensive had inflicted huge losses on the Austro-Hungarian armies and forced the Germans to take over more of the front. The declaration of war by Romania had placed additional strain on the Imperial German Army and the war economy.

The Siegfriedstellung (Hindenburg Line) built behind the Noyon Salient, was to replace the old front line as a precaution against a resumption of the Battle of the Somme in 1917. By devastating the intervening ground, the Germans could delay an Anglo-French offensive. A shortened front could be held with fewer troops and with tactical dispersal, reverse-slope positions, defence in depth and camouflage, German infantry could be conserved. Unrestricted submarine warfare and strategic bombing would weaken the Anglo-French as the German armies in the west (Westheer) recuperated. On 25 January 1917, the Germans had 133 divisions on the Western Front but this was insufficient to contemplate an offensive.

Alfred Peter Friedrich von Tirpitz (German pronunciation: [ˈalfʁeːt fɔn ˈtɪʁpɪt͡s] ; born Alfred Peter Friedrich Tirpitz; 19 March 1849 – 6 March 1930) was a German grand admiral and State Secretary of the German Imperial Naval Office, the administrative branch of the German Imperial Navy from 1897 until 1916.

Prussia never had a major navy, nor did any of the other German states before the German Empire was formed in 1871. Tirpitz took the modest Imperial Navy and, starting in the 1890s, turned it into a world-class force that could threaten Britain's Royal Navy. However, during World War I, his High Seas Fleet proved unable to end Britain's command of the sea and its chokehold on Germany's economy. The one great engagement at sea, the Battle of Jutland, ended in a narrow German tactical victory but a strategic failure. As the High Seas Fleet's limitations became increasingly apparent during the war, Tirpitz became an outspoken advocate for unrestricted submarine warfare, a policy which would ultimately bring Germany into conflict with the United States. By the beginning of 1916, he was dismissed from office and never regained power. Following his dismissal, he would become Chairman of the far-right German Fatherland Party, an ideological precursor to the German National People's Party.