The assassination of Archduke Franz Ferdinand, heir to the Austro-Hungarian empire led to the immediate outbreak of war among several nations in Europe which then turned into a Great World War.
Serbian nationalists wanted to end the Austro-Hungarian rule over the Balkan(mainly in Serbia, Bosnia, Herzegovina) region.
Archduke visited Bosnia on a military inspection tour. Serbians used this opportunity and killed Franz Ferdinand and his wife.
Austria confirmed to declare war on Serbia after consulting help from Germany to prevent Russian intervention.
Russian entered the war with France as they were allies to Serbia.
The dual monarchy of Austria-Hungary sent their final demands to Serbia in harsh terms which made Serbia impossible to accept. This gives an yellow signal to both military troops to get ready for war.
Germany sent its troops to Luxembourg and invaded Belgium demanding a free passage into France to declare war.
Great Britain which had no concern about the on-going war declared war on Germany to defend Belgium.
Peace between the European nations quickly collapsed and within a week Serbia, Russia, France, Belgium, Great Britain had lined up against Austria-Hungary and Germany. World War I had begun!
The outbreak of war in August 1914 was actually greeted with celebration and confidence by the people of Europe. Most welcomed the war with a patriotic feeling wishing their own country to be victorious anyway. Only few feared about the disastrous consequences and monstrous deaths the war going to bring.