How was nationalism a part of the cause of world war 1?

Nationalism is the concept that if you are part of a nation regardless of where you are you are responsible to that nations leadership. If the leadership tels you that you must go out and fight on their ( The Nations Behalf) you do so. If you question it especially at times of war this will be seen as unpatriotic and you could be executed for this. Nationalism ensures that differing groups of people are kept appart and can the be used in conflicts for the (so called) common good.