In June 1914, Austria’s Archduke Franz Ferdinand was assassinated in Sarajevo, Bosnia by a member of Serbia’s infamous Black Hand organization. By mid-August, war broke out between the nations of the Central Powers and Allied Powers. The Central Powers consisted of Austria-Hungary, Germany and the Ottoman Empire. The Allied Powers consisted of Great Britain, France, Italy, Japan and Russia.
In November 1914, Great Britain’s navy formed a blockade around Germany’s ports in the north, declaring the area a war zone. The blockade cut off the importation of many supplies including food. Germany responded by sending out their U-Boats to disrupt shipping into and out of Great Britain by declaring the seas around Great Britain to also be a war zone.
The U-Boat attacks were indiscriminate in that they sank war ships and commercial ships. On May 7, 1915, a German U-boat fired torpedoes at the passenger liner RMS Lusitania off the coast of Ireland. In less than 20 minutes, the Lusitania sank along with 1,198 of the 1,959 people on board, of which 128 were American civilians. Germany tried to justify the attack by claiming that the Lusitania was carrying military supplies and ammunition.
On March 24, 1916, a German U-Boat torpedoed the passenger ferry SS Sussex. Although the Sussex didn’t sink, at least 50 to 80 people died.
President Woodrow Wilson had vowed to keep the US out of the war and had managed to keep that promise for two years. He was running for re-election in 1916 and ran on the slogan: ‘He kept us out of war’. However, Wilson could not ignore the continued sinking of civilian ships and loss of civilians at the hands of the German U-Boats.
At the torpedoing of the Sussex, Wilson contacted German diplomats and threatened to take a variety of actions against Germany if they continued to attack civilian vessels. On May 4, 1916, America and Germany came to an agreement, called the Sussex Pledge in which Germany promised to give proper warning to all merchant and passenger vessels, giving them enough time to safely abandon ship before the Germans attacked.
The Sussex Pledge kept America out of the war long enough to help Wilson narrowly win re-election. However, less than a month before his second inauguration, Germany decided to abandon the Sussex Pledge and escalated their attacks on all ships in the declared war zone around Great Britain.
On February 3, 1917, Wilson informed Congress that he had severed all diplomatic relations with Germany.
In February 1917, German U Boats fired upon and sunk two American ships, the SS Housatonic and the SS California.
On February 24, 1917, British intelligence shared an intercepted telegram with President Wilson. The telegram was from German Foreign Minister Arthur Zimmerman to the German Embassy in Mexico City. The now infamous Zimmerman telegram revealed that Germany was offering to help Mexico regain territory they had lost to the US following the Mexican-American War if Mexico would help Germany in their war.
Wilson was still reticent about getting America involved with the war. However, on February 26, 1917, Wilson appeared before Congress and asked for their approval to arm US merchant ships with Navy guns and personnel. A group of anti-war senators argued against the measure so Wilson used an existing anti-piracy law to issue an executive order calling for the Navy to help arm and protect US merchant vessels sailing the Atlantic.
On March 20, 1917, Wilson felt he had no choice but to ask Congress for a declaration of war against Germany. He cited the Zimmerman telegram along with Germany’s abandonment of the Sussex Pledge and the sinking of the Lusitania and Arabic as reasons for war.
On April 4, 1917, The US Senate voted 82 to 6 in favor of a declaration of war with Germany.
On this day, April 6, 1917, the United States officially entered World War I when the US House of Representatives voted 373 to 50 in favor of a declaration of war.
Sources for the above includes: America Declares War on Germany, 1917; The United States Enters the War:; America enters World War I; The war that launched the American Century; U.S. Entered World War I April 6, 1917; U.S. Entry into World War I, 1917; The Sussex Pledge; World War I History; World War I.