Aggression: the action of attacking without provocation.
Modern nationalism exists in order to support nation states and their existence by defending them, often aggressively and by using military force, against individual outsiders or other nation states. Historically, such as during World War II, nationalist feeling - tipping over into racism - acted as a driver for large-scale militarisation, but now these large armies exist in precedent , they drive continued nationalism by behaving aggressively to those outside the nation state. These national armies, and the idea they represent of a nation state needing to defend itself against those who do not belong, suggest that nationalism is developed by the structure of modern nation states, which exist in opposition to others, and that nationalism and its aggressive tendencies are not organic.
Instances of aggressive nationalism have been evident throughout history, such as in the Greco-Persian wars, and suggesting that nationalism itself only exists as a result of modern nation states is disingenuous, even though modern nation states do encourage a new form of large-scale aggressive nationalism in terms of mechanised warfare.
Aggressive nationalism benefits those in power, because it allows them to maintain their power through encouraging citizens to defend their state, whether for a real or imagined reason.
Modern nationalism is aggressive because it exists to support nation states. Nation states are set up in opposition to each other, making nationalism necessary for their survival. Nationalism and its aggressive tendencies are therefore created by nation states.
Rejecting the premises
Instances of aggressive nationalism can be seen throughout history. On some level, all different groups and nations exist in opposition to each other. Nationalism has always existed and has always been aggressive.