Germany must take the major blame for the outbreak of WW1, for the following reasons:
Firstly, unconditional support for Austria during the July Crisis of 1914.Austria wanted revenge against Serbia for the assassination of Franz Ferdinand,but was aware that Russia was likely to support Serbia as brother Slavs and Eastern Orthodox Christians.Russia was too strong for Austria to fight alone - they needed the help of their ally, Germany.The Kaiser agreed to fully support Austria in all their demands against Serbia, even if it meant war with Russia, and even though the Kaiser himself thought Austria's demands on Serbia went too far.
Secondly, the German war plan. Russia was allied with France, so this would mean a 2 front war for Germany.The Schlieffen Plan was the German war plan to deal with this.Because of the vast distances involved and the relatively underdeveloped Russian transport system, it would take at least 2 months for the Russian army to fully mobilize and be ready to attack Germany. The Schlieffen Plan called for the German army to launch an immediate attack on France through neutral Belgium (something no one would expect).The German army would then outflank the French, surround and capture Paris, and force a French surrender.The Schlieffen Plan called for the capture of Paris within 6 weeks, leaving 2 weeks at least for the Germans to then transfer their army east to fight the Russians.However, the Schlieffen Plan would automatically bring France into the war; the Franco-Russian alliance was defensive, so there was no absolute guarantee of French help for Russia if Russia attacked Austria or Germany first. Further, an invasion of Belgium risked bringing Britain (guarantor of Belgian neutrality under the 1839 Treaty of London) into the war, against Germany. As this would involve the British navy, and because Britain, Germany, France and Belgium all had overseas colonies, this would spread the war to these places.So, only Germany had a war plan that would turn a localized,Balkan conflict into a Europe wide and global one.
Finally, the German army mobilization timetable. The main staging area for the German army was Aachen, on the border between Germany,France,and Belgium.However, Aachen was not large enough to accommodate the whole German army; the first troops to arrive would have to move out straight away to make room for the next wave of arrivals, and so on. Otherwise, the whole army could become snarled up in a giant traffic jam, unacceptable with war imminent.The only direction German troops in Aachen could move was forwards, into France and Belgium, thus starting war against both. Therefore mobilization of the German army meant war within 48 hours, something both the Kaiser and the German Army High Command were well aware of. Austria declared war on Serbia 28 July; Russia announced mobilization of its army (NOT a declaration of war) 31 July. The Kaiser then declared war on Russia and mobilized the German army, 1 August,knowing the first German troops would cross the Belgian and French borders 3 August.
So, Germany was most guilty for starting WW1 for the reasons given above.