A long diplomatic crisis marked July of 1914. With the assassination of Archduke Franz Ferdinand, heir apparent to the throne of Austria-Hungary, by Gavrilo Princip, a Bosnian Serb, Austria-Hungary turned aggressive and served Serbia a harsh ultimatum. Serbia responded to the ultimatum on July 25, 1914. Though all except two terms were accepted, Austria-Hungary seized the opportunity to declare war, assured of Germany’s support in case of a military crisis.
War was declared on July 28, 1914. Kaiser Wilhelm II, the German monarch, had not anticipated a war of the scale that ensued. Russia’s mobilization of troops threatened Germany who anticipated having to face a combined attack of Russia, France, and Britain.
Germany entered the fray on August 1, 1914, by declaring war on Russia, and on France on August 3. Germany’s invasion of Luxembourg and Belgium brought Britain into the war. With Britain, a number of countries from Asia, Africa, Australia, and Europe joined as part of the British Empire. Italy and the United States, initially neutral, joined World War I in 1915 and 1917 respectively.
Read more about the Declarations of War by Each Country
With the United States, a number of other countries joined the Allied cause. World War I thus became a truly global conflict with far reaching impact.
The map of Europe was largely redrawn at the end of World War I. The invasions of the Central and Allied troops, the dissolution of the Ottoman Empire and Austria-Hungary, and the creation of new states, consequent to the peace treaties, saw significant territorial redistribution. Read more about the First Invasions of World War I
Major Treaties of World War I