The Germans were merely keeping to their alliances and following their promises as Austria-Hungary declared war on Serbia, triggering the first World War due to alliances. The reason this all started in the first place was because of the assassination of Franz Ferdinand and his wife, the Duke and Duchess of Austria-Hungary by a Serbian patriot Gavrilo Princip. He was part of a group that wanted Bosnia and Herzegovina to be part of Serbia as they were more closely related to the Serbians than they were the Austro-Hungarians. He also assassinated them because Bosnia and Herzegovina is where Serbians have a lot of relatives.
WWI was called that because Britain was pulled into the war through France as France was pulled in by Russia. All the colonies participated because of their relationship to their mother country. As more and more people got involved, Austria-Hungary and Italy pulled out, so when it was over, the only person who could've been blamed for the whole catastrophe was Germany, even though he was just trying to defend one of his allies.