The two Germanic nations, Austria and Germany, were the first to declare war.
Russian backed Serbs murdered the heir to the throne of Austria-Hungary.
Austria mobilized against the Serbs.
Russia mobilized against Austria.
Germany partially mobilized against Russia.
France mobilized against Germany.
Once millions of men began moving all across the continent war became for practical purposes inevitable.
Who first declared war is moot.
The railroad time tables dictated the course of events.
Failure to mobilize in time meant defeat for the dilatory power.
George F. Kennan held France and her desire for revenge on Germany to be the power most responsible for setting in motion the events leading to WWI.
The Kaiser was probably the single individual most responsible for upsetting the delicate diplomatic structures Bismarck had created to maintain the balance of power on the continent and for pursuing a naval arms race with Britain which Germany could not win but that led England to abandon neutrality in search of security.