This depends greatly on what historians can agree on to be the important chapters of WWI -- whether it be the very declarations of war, the introduction and application of certain technologies or military tactics on the battlefield, political backgrounds, and so on.
I see the war happening in FIVE distinct parts, with the appropriate subchapters.
The Assassination of the Archduke Franz Ferdinand
July 28th, 1914: Austria-Hungary declares war on Serbia
July 31st: Russia enters war/unconditionally allies with Serbia
August 1st: Germany mobilizes her troops against Russia
August 3rd: Germany declares war on France ... etc.
The Eastern and Western Fronts Form
August 4th: Germany declares war on Belgium/Schlieffen Plan
August 22nd: 27,000 French troops are killed in a single day
August 26th: Germany gains victory at Tennenberg ... etc
Trench Warfare Becomes a 4-year Reality
September 15th: First trench dug on Western Front
February 21st - December 18th, 1916: Longest battle of the war (Battle of Verdun), which resulted in an estimated 1 million casualties
May 31st - June 1st: Battle of Jutland
July 1st - November 18th: Battle of the Somme, also results in near 1 million casualties ... etc
The U.S. Becomes a Belligerent, Russia Withdraws
April 6th, 1917: Pres. Woodrow Wilson asks Congress to declare war on Germany
December 3rd: Leon Trotsky and the new Russian government sign armistice with Germany ... etc
Wrapping Up the War
March 21st, 1918: Germany advances in an all-or-nothing attack to win the war before Americans appear on the battlefield; offensive is a failure
August 8th: Americans push Germany out of The Somme
November 11th: Allied forces and Germany sign Armistice
The pre-war and post-war events are also important, and you should read (or watch) about them to learn this war's context in history. So many lives were lost for so little.