Russia and France had a treaty declaring if one nation went to war, the other would respond. Germany and Austria/Hungry had a similar treaty. Serbian terrorists murdered the almost leader of Austria/Hungry. Austria/Hungry went to war with Serbia. Serbia and Russia were allies so Russia declared war on Austria/Hungry. Germany attacked France because of the aforementioned treaty. England attacked Germany because Germany invaded Belgium to attack France.
Believe me there is way more to it than that. The origins of WW1 are unbelievably complex. Literally thousands of books have been written on this subject.
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