1914
- June 28
- Archduke Francis Ferdinand is assassinated.
- July 28
- Austria-Hungary declares war on Serbia, beginning World War I.
- August 2-7
- Germany invades Luxembourg and Belgium. France invades Alsace. British forces arrive in France. Nations allied against Germany were eventually to include Great Britain, Russia, Italy, Australia, New Zealand, South Africa, Rhodesia, Romania, Greece, France, Belgium, United States, Canada, Serbia, India, Portugal, Montenegro, and Poland.
- August 10
- Austria-Hungary invades Russia.
- September 9
- Allied forces halt German advance into France during First Battle of the Marne.
1915
- February 18
- Germany begins naval blockade of Great Britain.
- April 25
- Allied forces land on the Gallipoli Peninsula of the Ottoman Empire.
- May 7
- German submarine sinks the passenger liner Lusitania during crossing from New York to Liverpool, England, killing 128 Americans.
- May 23
- Italy declares war on Austria-Hungary.
1916
- February 21
- Germany begins the attack on Verdun.
- May 31
- Naval Battle of Jutland takes place between British and German fleets.
- July 1
- Allied offensive begins the Battle of the Somme.
- December 18
- Battle of Verdun ends with 550,000 French and 450,000 German casualties.
1917
- February 1
- Germany returns to unrestricted submarine warfare halted after the sinking of the Lusitania.
- February 3
- United States severs diplomatic relations with Germany.
- April 6
- The United States declares war on Germany.
- June 7
- General John J. Pershing, newly selected commander of the American Expeditionary Forces (AEF), arrives in England with his staff.
- June 24
- American combat forces arrive in France.
- December 15
- Russia signs armistice with Germany.
1918
- January 8
- President Woodrow Wilson presents to Congress his outline of Fourteen Points required for peace.
- March 3
- Russia signs the Treaty of Brest-Litovsk with Germany.
- March 21
- Germany begins its final offensive of the war.
- March
- American women recruited to serve as bilingual telephone operators for the AEF arrive in Europe.
- May 28
- United States forces are victorious in the Battle of Cantigny, the first independent American operation.
- June 2
- American forces stop German attempt to cross the Marne River at Chateau-Thierry.
- September 12
- American First Army attacks St. Mihiel salient.
- September 26
- Allied forces begin the attack at Meusse-Argonne, the final offensive of the war.
- November 11
- Germany signs the Armistice at Compiègne, ending World War I.
- December
- Harold Ross assumes editorship of The Stars and Stripes.
- December 1
- British and American forces enter Germany.
1919
- Jan. 18
- Peace conference begins at Paris.
- February 14
- Draft of the covenant of the League of Nations is completed.
- June 28
- Allied and German representatives sign treaty of Versailles. The United States signs treaty of guaranty, pledging to defend France in case of an unprovoked attack by Germany.
- November 19
- United States Senate fails to ratify Treaty of Versailles.
- January 10
- Treaty of Versailles takes effect.
1920
- March 19
- United States Senate fails to ratify Treaty of Versailles for the second time.
1921
- August 24-29
- United States signs separate peace treaties with Germany, Austria, and Hungary.