The First World War begun on July 28th 1914 when Austria-Hungary declared war on Serbia click here to read my post on the build up of the First World War.
On 3rd November 1914 the North Sea was declared a British military area, the Royal Navy patrolled the sea stopping thousands of foreign merchant ships which were destined for Germany and escorted them into British ports to be searched, if supplies were found which could assist the German war effort they were seized. Germany responded to the blockade by torpedoing merchant and passenger ships belonging to the Allied Powers and even neutral countries if spotted near the British Isles. In 1915 both British passenger liners RMS Lusitania and RMS Arabic were sunk by German U-boat torpedoes killing more than 1,000 people 131 of them Americans. In March 1916 a German U-boat torpedoed the French boat the Sussex, USA President Woodrow Wilson threatened to sever diplomatic relations with Germany if they continued to torpedo unarmed boats, the Germans agreed to stop on the 4th May 1916, this is known as the Sussex pledge.
On 19th January 1917 British codebreakers intercepted and decrypted a telegram sent by German Foreign Minister Arthur Zimmerman to the German Ambassador in Mexico City Heinrich von Eckhardt, Germany proposed to Mexico that they join the Central Powers as an ally and in return Germany would help Mexico recover the territory it had lost to the USA following the Mexican-American War. The British didn’t share this information with the USA straight away because they didn’t want the Germans to know they had cracked their code. On the 1st February 1917 the Sussex Pledge was violated by Germany who resumed their practice of unrestricted submarine warfare, they believed the USA had jeopardized their neutrality by not intervening with the Allied blockade of Germany and supplying arms and money to the Allied Powers. Prior to this decision German Chancellor Theobald von Bethmann-Hollweg objected claiming the USA would join the war against Germany which would lead to the defeat of the Central Powers, however his protests were ignored.
On 3rd February 1917 upon hearing the news that Germany resumed their practice of unrestricted submarine warfare President Wilson severed diplomatic relations with Germany however he didn’t declare war on Germany as he was unsure the USA public would back him. During February and March 1917, German U-boats torpedoed American ships killing more innocent people. Finally on 24th February 1917 the British sent the Zimmerman Telegraph to USA President Woodrow Wilson, this news was passed on the American media and published on 1st March 1917 the news of the Zimmerman Telegraph swayed the majority of the American public opinion to join the war.
All of the above was enough to convinced Woodrow Wilson to declare war on Germany on 6th April 1917.
US Department of State – American Entry into World War I link
The National Archives – The Blockade of Germany link