1 February 1917: Germany Announces Resumption of Unrestricted Submarine Warfare

Preliminaries

Germany only had about 28 U-boats available in the early months of the war, but they achieved some spectacular successes:

• 5 September 1914: U-21 sinks Royal Navy light cruiser Pathfinder — first combat victory of the modern submarine.

• 22 September 1914: U-9 sinks three British armored cruisers, Aboukir, Cressy, and Hogue.

Triumphant U-9 in Harbor

A push came from within the German Navy to wage unrestricted submarine warfare.

The gravity of the situation demands that we should free ourselves from all scruples.

German Admiral Frederich von Ingenohl, November 1914

It has been demonstrated that our submarines have not succeeded now for a long time in gaining any results worthy of note, despite the fact that they have been making cruises for a long time and have carried them out with great boldness. In the future prosecution of the war we will therefore be able to count neither on an equality of strength before the battle due to the use of our light forces nor on the opponent's changing his strategy as long as we continue ours unchanged.

Kapitän zur See Zenker, 28 December 1914

First Declaration

The waters round Great Britain and Ireland, including the English Channel, are hereby proclaimed a war region. On and after February 18th every enemy merchant vessel found in this region will be destroyed, without its always being possible to warn the crews or passengers of the dangers threatening.

Admiral Von Pohl, Chief of Marine Staff, Berlin,  4 February 1915

• At this time the German Navy has 30 U-boats available, an increase of two since the war's opening.

• 7 May 1915: RMS Lusitania sunk off of Queenstown, Ireland, by U-20. Of the 1,195 lives lost, 128 were U.S. citizens. The ensuing controversy brought unrestricted submarine warfare to a temporary halt, but contributed to the rise of American sentiment for entering the war against Imperial Germany.

Burial of American Victim of Lusitania Sinking

Unrestricted Submarine Warfare Resumes in February 1917:

All shipping, enemy and neutral, including passenger ships, would be liable to attack without warning in war zones. The principal war zones included the waters encompassing the British Isles, including the Channel, the western half of the North Sea, and the waters extending 400 nautical miles from the west coast of France. Also declared a war zone was the entire Mediterranean Sea with the exception of Spanish coastal waters and a lane, 20-nautical miles wide, set aside for Greek steamers. In March, the Barents Sea was added to the list. Further expansions, encompassing broad swaths of water around the Azores and Canary Islands and, eventually, most of the North Atlantic, were declared in November 1917 and January 1918.

• At this time the German Navy has 142 U-boats available, an increase of 114 since the war's opening.

• 6 April 1917: The United States declares war on Germany, which was expected by the German leadership.

• 860,334 tons of Allied shipping sunk in April 1917 — greatest one-month losses of the war.

The unprecedented losses suffered during the last fortnight of April [1917], especially in the approach areas, greatly strengthened the hands of those who advocated the general introduction of the convoy system...  

Seaborne Trade, Ernest Fayle.

We Are Losing the War!

Admiral William Sims, U.S. Navy, April, 1917

• May 1917: The convoy system is implemented, eventually reducing the losses markedly, but never eliminating them entirely.

American Troops En Route to France Via Convoy

• In October 1918, last full month of the war, losses were 116,237 tons. At the Armistice, the German Navy had 134 boats in service with 229 more under construction.