There is no way to objectively answer this question. Various countries of Europe were at fault to some degree and there is no way to really measure the degree of fault for any country.
The most common answer to this question is Germany. Germany was the European country that was least satisfied with the status quo at the time of the war. Germany felt that it deserved its “place in the sun.” It felt that it was not sufficiently powerful and that it did not have enough prestige. Therefore, it took various actions, like increasing the size of its navy, that frightened other countries and made them prepare for war. If Germany would only have been satisfied with its level of power and prestige, the war need never have happened.
Another answer could be that France, Russia, and England were to blame. These were the major countries who felt threatened by Germany’s increasing strength. With Germany’s power increasing, these countries tightened their alliance and decided they needed to act as a check on Germany. If these countries had simply chosen not to feel threatened by the rise in German power, the war need never have happened.
It is not possible to say in any objective and scientific way which of these scenarios is truer. One can blame either side or one can even simply blame no one and say that the war was inevitable given the conditions in Europe at the time.