1917
- January 17 - The United States pays Denmark $25 million for the U.S. Virgin Islands.
- January 22 - World War I: President of the United States|President]] Woodrow Wilson calls for "peace without victory" in Europe.
- January 25 - The Danish West Indies is sold to the United States for $25 million
- January 28 - The United States ends search for Pancho Villa
- January 31 - World War I: Germany announces its U-boats will engage in unrestricted submarine warfare.
- February 3 - World War I: The United States breaks off diplomatic relations with Germany a day after Germany announced a new policy of unrestricted submarine warfare.
- February 5 - The constitution of Mexico is adopted.
- February 23 - The Russian Revolution begins with the overthrow of the Tsar system.
- February 24 - World War I: United States ambassador to the United Kingdom Walter H. Page is given the Zimmermann Telegram, in which Germany offers
* March 21 - The Danish West Indies become the Virgin Islands when Denmark transfers control over the islands to the United States after the purchase of the islands on January 25.
March 26 - World War I: Battle of Gaza - British cavalry troops retreat after 17,000 Turks block their advance.- March 31 - The United States takes possession of the Virgin Islands after paying $25 million to Denmark.
- April 2 - World War I: US President Woodrow Wilson asks Congress for a declaration of war on Germany.
- April 6 - World War I: United States declares war on Germany.
- April 9 - 12 - World War I: Canada wins the Battle of Vimy Ridge.
- April 11 - World War I: Brazil severed relations with Germany.
- May 13 - Three peasant children claim to see the Virgin Mary above a holmoak tree in Cova da Irianear near Fatima, Portugal.
- May 18 - World War I: The Selective Service Act passes the United States Congress giving the President the power to draft soldiers.
- June 15 - The United States enacts the Espionage Act, which, when combined with the later Sedition Act, suspends constitutional freedom of speech.
- July 17 - King George V of the United Kingdom issues a Proclamation stating that the male line descendants of the British royal family will bear the surname Windsor.
- July 25 - Sir Thomas Whyte introduces the first income tax in Canada as a "temporary" measure (lowest bracket is 4% and highest is 25%).
- October 15 - World War I: At Vincennes outside of Paris, Dutch dancer Mata Hari is executed by firing squad for spying for Germany.
- October 26 - World War I: Brazil declared in state of war with Germany.
- November 6 - World War I: Third Battle of Ypres ends: After three months of fierce fighting,Canadian forces take Ypres in Belgium.
- November 20 - World War I: Battle of Cambrai begins - British forces make early progress in an attack on German positions but are soon beat back.
- December 3 - After nearly 20 years of planning and construction, the Quebec Bridge opens to traffic (the bridge partially collapsed on August 29, 1907 and September 11, 1916).
- December 6 - Belgian Ship Imo collided with French ship Mont Blanc at Halifax. Mont Blanc was loaded with ammunition and caught fire; eventually exploding at 9:06am, taking most of Halifax with it. Until Hiroshima, this incident was the biggest manmade explosion ever.