June 28, 1914 – Bosnian-Serb activist Gavrilo Princip assassinated Archduke Franz Ferdinand, heir to the Austro-Hungarian throne.

July 28, 1914 – Austria-Hungary declared war on Serbia.

Aug. 1, 1914 – Germany, Austria-Hungary's strongest ally, declares war on Russia, Serbia's closest ally.

Aug. 3, 1914 – Germany declares war on France.

Aug. 12, 1914 – Battle of Cer was the first Allied victory.

Aug. 14-24, 1914 – In the Battle of the Frontiers, Germany advanced into Belgium and France.

Aug. 17 – Sept. 2, 1914 – In the First Battle of Tannenberg, Germany defeated Russia as it moved westward.

Sept. 5-12, 1914 – In the First Battle of the Marne, French and British forces halted Germany's advance into France.

Feb. 15, 1915 – Germany declared a war zone around the British Isles so that even neutral nations' vessels were subject to attack.

April 1915 – Italy joined Allied forces.

April 1915 – Germany issued first large-scale chlorine gas attack, in violation of international treaties, against the French in the Second Battle of Ypres.

May 5, 1915– Central Powers broke into Poland.

May 7, 1915 – Germany sank the British Lusitania passenger ship, killing 1,198 of the 1,959 people on board. One hundred and twenty eight of the 197 Americans on board were killed. U.S. President Woodrow Wilson issued a warning to Germany, which led U.S. Secretary of State William Jennings Bryan, a firm neutralist, to resign.

Aug. 8, 1915 – Germany captured Warsaw in "Great Advance" and caused Russia's "Great Retreat."

October 1915 – U.S. recognized Venustiano Carranza as the new president of Mexico and fellow revolutionary Francisco "Pancho" Villa started a civil war with his raids across the country. Villa's raid into New Mexico killed 17 U.S. citizens and caused Wilson to send Gen. John J. Pershing in charge of a "The Punitive Expedition" to Mexico.

November 1915 – Serbia fell to Austria-Hungary after being invaded by Slovenia, Croatia and Bosnia.

January 1916 – Congress passed the National Defense Act in response to American concern over military unpreparedness. The law appropriated money to expand the Army and Navy, allowed the National Guard to be called up for federal service, and created the Reserve Officer Training Corps.

April 1916 – Newton D. Baker became the U.S. secretary of war. The former mayor of Cleveland had no military experience.

May 31- June 11, 1916 – The Battle of Jutland in the North Sea was the largest naval fight of the war with a full-scale clash of battleships. Like many WWI battles, there was no clear victor as it was considered a standoff whereby Britain asserted its control of the sea, but at huge costs in lives and ships.

June & July 1916 – In the Brusilov Offensive, Russian Gen. Aleksey Brusilov achieved victory by leading a surprise attack against the Austro-German line in two places.

June 1916 – In the Battle of Mecca, an Arab revolt defeated the Ottoman Empire with Ottoman's surrender of Damascus.

July 1, 1916 – Great Britain suffered 57,470 casualties and 19,240 dead at the Battle of the Somme.

August 1916 – Romania dropped its long-held neutrality and declared war on Germany and Austria-Hungary.

Dec. 12, 1916 – Following a crushing defeat of Romania in the Battle of the Arges River, German Chancellor Theobald von Bethmann-Hollweg proposed a peace agreement that would have annexed portions of France and Belgium to Germany, which the Allies soundly rejected.

Dec.18, 1916 – President Wilson invited Allies and Central Powers to peace talks in which they were to state their "war aims." The talks collapsed when it became clear to the Central Powers that the U.S. was colluding with the Allies.

January 1917 – Wilson tried again at mediation with a suggestion of "peace without victory," which was accepted by the Allies and Austria-Hungary. Germany balked, however, and returned to a policy of unrestricted submarine warfare.

Feb. 3, 1917 – Wilson severed diplomatic ties to Germany after it sank the US warship, the Housatonic.

March 1917 – British intelligence intercepted a telegram from German Foreign Minister Alfred Zimmermann (Zimmerman Telegram) to Mexico to join the war as Germany's ally against the U.S. with Japan as Mexico's ally. Zimmermann was attempting to delay U.S. entry into Europe and promised Mexico that Germany would help them win back Texas, Arizona and New Mexico.

March 1917 – Great Britain captured Baghdad.

March 1917 – Internal unrest in Russia causes the abdication of Tsar Nicholas II in the start of the Russian Revolution.

April 6, 1917 – Congress approved Wilson’s request and declared war on Germany, becoming an "associated power," rather than a member, of the Allies.

June 1917 – The Battle of Messines, at Ypres, caused 25,000 casualties for Germany and 17,000 for Britain after Britain successfully drove Germans from the area.

July 15, 1917 – The first successful allied offensive was The Second Battle of Marne when Germans attempted to encircle Reims.

August 1917 – Battles in Flanders and at Verdun caused more than a quarter of a million deaths.

February 1917 – Congress imposes citizenship on Puerto Ricans so they can be drafted.

October 1917 – Vladimir Lenin and Leon Trotsky led the second phase of The Bolshevik Revolution that transformed Russia into a communist state.

Nov. 5, 1917 – A Supreme War Council of Allied forces was created at the Doullens conference.

November 1917 – Vladimir Lenin and the Bolshevik Party assumed power in Russia.

December 1917 – Russia surrendered to Germany and ceded Finland, Baltic provinces, parts of Poland and Ukraine to the Central Powers.

February 1918 – Establishment of Democratic Republics of Armenia and Georgia.

March 21, 1918 – The Germans led a successful offensive on the Western Front known as Operation Michael, against the U.S., British and French near Amiens, France.

April 20, 1918 – The first major U.S. battle occurred at Seicheprey, along the Lys River near the Belgian-French border. It was a successful German attack that left 669 Americans dead or wounded, while fighting in support of the British.

May 28, 1918 – U.S. Maj. Gen. Robert Lee Bullard launched the first U.S. offensive of the war at the French village of Cantigny, 60 miles north of Paris, at a heavily-fortified German observation point. The 1st Division, or "Big Red One," drove out the Germans over two days of fighting.

May 1918 – U.S. and French forces drive out Germans from France’s Marne River outside of Paris.

Aug. 8, 1918 – The Start of 100 Days Offensive, British, French and Canadians advance into German territory in Battle of Amiens, taking more than 100,000 German soldiers prisoner and marking the beginning of the end for Germany.

Sept. 29, 1918 – Bulgaria surrendered to Allies.

October 1918 – The Ottoman Empire surrendered to the Allies, Budapest, Prague and Zagreb declared independence and the Austro-Hungarian Empire fell.

Nov. 11, 1918 – After suffering six million casualties, Germany signs an armistice with the Allies. Wilson demanded the abdication of the kaiser, ending imperial Germany, which was replaced with the democratic, Weimar Republic.

June 28, 1919 – Signing of the Treaty of Versailles.