Germany alone is responsible
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Germany coerced their ally, Austria-Hungary, to begin conflict
Without the coercion from Germany, Austria never would have gone through with retaliation following the Assassination. As the only war to have been caused by assassination, Germany was certainly whispering to provoke an overreaction.
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Bethmann-Hollweg, Chancellor of Germany, essentially handed Austria-Hungary a "blank cheque" following the initial insurrection caused by the assassination of Archduke Ferdinand.  This gesture signified to Austria that if they choose a path of conflict, Germany would support them and provide limitless resources to them in order to comfortably win and secure political triumph by fighting for the Balkans.  Additionally, by providing this complete and absolute support for their ally, Germany could virtually advocate for themselves. They could insert their interests into a smaller conflict through offering support, but then expand it into a mass conflict of greater stakes and opportunity for both themselves and Austria-Hungary.
It can be argued that Germany persuaded Austria, but this does not change the role of Austria in the war and the state of affairs between Austria and Serbia. Without the Austrian/Serbian conflict, the War would not have incited and employed as many world powers as it had. Therefore, Germany can not be named the sole responsible actor.