cookiecutter
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I would definitely dispute that the reasons wars became bloodier with the rise of the nation state is due to a sense of a national identity. It is due to tanks and machine guns and bombers and things like that. Really the only major conflict that has been fought since the development of the nation-state was WWII and as we've already said that grew out of WWI, a conflict you yourself admit was started by the European empires.There were lots of wars before the development of nation-states, but they were generally fought by mercenaries and involved relatively little bloodshed and a lot of maneuvering. Wars certainly became much bloodier with the development of nation-states, since there were people who were willing to fight to the death. However, I think the root here is not the development of nation-states, but the development of a conscious sense of national or ethnic identity.
Serbia indeed was a nation-state at the outbreak of WWI but they were merely the trigger as you say not the underlying cause. If not for the network of alliances engineered by the French, German, Russian, British and Austro-Hungarian empires, it would've merely been a regional conflict.
Prior to 1914 there were tons of conflicts. If you want bloody examples look at the Boer War or the British invasion of Tibet. I know this all stemmed from a humorous cartoon, but I don't think the argument that the creation of nation-states has made humanity more warlike is not defensible.