Nationalism in countries such as Ireland played a vital role in the outbreak of WW1. The British, who were in control of Ireland, told the Irish Nationalists that they may get Home Rule (semi-independence) if they joined the British Army. Many did, but an Irish rebellion took place in 1916 which proved to undermine the Nationalists and ultimately led to a Republic of Ireland (today's Ireland). This type of story is applicable to many empires of WW1, such as the Austrian empire, who promised similar concessions for Hungarian and Czechoslovakian Nationalists if they fought along side them.
Ultimately Nationalism was manipulated by the great powers of Europe.
(I have personal insight as my great grandfather fought for Irish Nationalism)
