the main reason was the power vacuum left by the recession of turkish control to southeastern thrace.  the great powers wanted to exert their hegemony over the balkans and wrest final control of constantinople and the straits. the archdukes death was austria-hungarys excuse to impress itself upon serbia. austria-hungary fucking with serbia was russias excuse to get involved in the mess. germany used russias response as an excuse to get involved. france used germanys response as an excuse to get involved. once germany rolled through belgium, that was britains excuse and it all kind of spitballed from there
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