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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.
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The assassination of Archduke Franz Ferdinand in Sarajevo.
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Austria-Hungary threatened war on Serbia after assassination. Germany sided by Austria-Hungary, while Russia sided with the Serbians.
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Austria-Hungary declared war on Serbia.
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Germany then declared war on Russia to help Austria-Hungary.
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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.
