It does not seem likely that the war could have been prevented. Wilhelm II had surrounded himself with war inclined military advisers, had himself entered a very militant mood by then and was determined to go into the war to expand the German Empire.
Otto von Bismarck, who had worked hard in preceding decades to maintain the diplomatic balance and to stop the German Empire from entering a new war repeatedly warned the Kaiser (Wilhelm II) of a looming disaster if the Kaiser were to proceed with any war plans. Alas, Otto von Bismarck and Wilhelm II had a massive fallout, the Kaiser continued to crave more place under the sun for the empire, and, after the former chancellor resigned and retired, Otto von Bismarck's advice was all but quickly ignored.
There is an interesting telegram exchange[0] between the Russian and German emperors, Nicholas II and Wilhelm II, who were cousins and referred to each other as Nicky and Willy, known as «Willy–Nicky telegrams». They corresponded with each other in English. One telegram dated with the 29th July 1914 is particularly interesting as Nicholas II offered a reconcilliation chance to Wilhelm II after the assassination of Franz Ferdinand:
«Thanks for your telegram conciliatory and friendly. Whereas official message presented today by your ambassador to my minister was conveyed in a very different tone. Beg you to explain this divergency! It would be right to give over the Austro-servian [sic] problem to the Hague conference. Trust in your wisdom and friendship. Your loving Nicky»
Wilhelm's unreasonable stubborness and unwise decisions even led to Adolf Hitler scornfully dispising Wilhelm later and openly calling Wilhelm an idiot.