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alliances
There were a lot of defense alliances between the major European countries. So if one country declare war on another country then all the countries defense allies would declare war on one country. -
assassination
The assassination of Archduke Franz Ferdinand in Sarajevo. -
threats
Austria-Hungary threatened war on Serbia after assassination. Germany sided by Austria-Hungary, while Russia sided with the Serbians. -
WAR BREAKS OUT
Austria-Hungary declared war on Serbia. -
MORE WAR
Germany then declared war on Russia to help Austria-Hungary. -
EVEN MORE WAR
German troops marched to France and the route went through Belgium. Since Britain had agreed to maintain the fairness of Belgium, so they immediately declared war on Germany.