The sinking of the Lyman M. Law

12 February 1917 A four-masted U.S. schooner the Lyman M. Law, carrying a cargo of wood from Stockton, Maine to Palermo, Italy was stopped and sunk by the German submarine U-35 early on a bright, sunny Monday morning. The Lyman M. Law was about 25 miles off the coast of Sardinia and near the port of Cagliari when she was attacked. The Germans allowed the crew to abandon ship before sinking the Law. (see http://uboat.net/wwi/ships_hit/3777.html ). This was the second U.S. merchant vessel sunk by the Germans since the resumption of unrestricted submarine warfare on 1 February.

Zimmermann Telegram: the last piece falls in place

10 February 1917 British Director of Naval Intelligence Admiral Hall receives a copy of the Zimmermann Telegram that was stolen by a British agent from a telegraph office in Mexico City. The last piece of the puzzle, this provides the copy that Admiral Hall can give to the Americans and hide the fact that the British intercepted and decoded the original. 

Because this version of the telegram (from Count von Bernstorff in Washington to German Envoy Heinrich von Eckardt in Mexico City) is encrypted using the older 13040 German diplomatic code, Hall can safely show it to the Americans without revealing the British are now partially breaking the new 0075 code.

Zimmermann worried; Pershing leaves Mexico; Hall delivers the Telegram

5 February 1917 German Foreign Minister Alfred Zimmermann telegraphs German Resident Minister Heinrich von Eckardt in Mexico City again, urging him to conclude an arrangement with the Mexican government immediately. 

On the same day, the last American troops of the "Punitive Expedition," led by General John J. Pershing, leave Mexico. 

In London, British Director of Naval Intelligence (DNI) Admiral Sir William Reginald Hall is finally ready and takes the now mostly decrypted Zimmermann telegram across the street from Room 40 to the Foreign Office and delivers it to Lord Hardinge, the Permanent Under-Secretary for Foreign Affairs.

U.S. Severs diplomatic relations with German Empire

3 February 1917 The U.S. severs diplomatic relations with Germany after the sinking of the American steamship Housatonic. In the next two months the Germans sink over 500 ships in the Atlantic, including 6 American ships, two in February and 4 in March. Their plan is to starve the British and get them to sue for peace within 6 months, leaving the Germans free to just fight the French.  

Ambassador von Bernstorff is called to the State Department late in the afternoon on 3 February and Secretary of State Lansing hands him back his passport and orders the embassy closed and all the staff expelled. Within 2 days von Bernstorff takes ship back to Germany, continuing to defend his efforts at peace. The U.S. and Germany edge closer and closer to war.

Germans resume unrestricted submarine warfare

1 February 1917 The Germans resume unrestricted submarine warfare in the Atlantic. Two days later an American steamer, the Housatonic is sunk in the north Atlantic, 20 miles south of Bishop Rock off the southwestern coast of England by the U-53. (All hands survived.) The Housatonic was carrying a cargo of wheat.

Germany's Warning - too little too late

31 January 1917 German Ambassador Heinrich von Bernstorff visits American Secretary of State Robert Lansing late on the afternoon of Wednesday 31 January and informs him of the resumption of unrestricted submarine warfare by the German Empire  8 hours before it is to start.

Peace without victory - Wilson's last try

22 January 1917 President Wilson delivers his famous "peace without victory' speech to Congress. It is his last plea to the belligerents in Europe to stop the fighting. Within 9 days events beyond Wilson's control will intensify the push of the US into the conflict in Europe.