Nationalism and Alliance systems played a major role in the outbreak of WW1 because most countries had signed military pacts within Europe and they had to honor such pacts.
The first world war was a conflict which involved Europe, America, Asia and Africa.
The war was caused primarily by the assassination of Franz Ferdinand of Austria in Serbia.
This assassination resulted in the conflict between Austria-Hungary and Serbia and other countries were drawn into the war because of earlier alliances formed.
Nationalism played a great role during WW1. Most actors during the war were nationalist and each had to defend their honour.
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