Serbians kill him
This "blank check" tells Austria-Hungary that they can do what they want to get revenge on Serbia, and Germany will support them.
Austria sens Serbia a list of demands to which Austria knows they will not concede. This gives Austria an excuse to fight them
Alliance between Russian, Britain, and France
Alliance between Germany, Austria-Hungary, and Italy
leads to Armenian massacre
German orders Russia to stop military preparations in 12 hours or they will go to war
Germany launches unrestricted submarine warfare on British coast, which kills many American traders, leading to America joining the war