Ironically, WWI was caused by A LACK OF NATIONALISM; The tinder box of Europe was caused by all the regional powers making alliances against what proved to be their nationalist interest. Why should England war against Germany? Why should a radical band of terrorists and anarchists in Serbia be anyone’s interests but Austria-Hungary?
Britain needed and guaranteed a neutral Channel coast. When Germany invaded neutral Belgium, that neutrality was threatened, and Britain declared war.
WW 1, as you correctly stated, was caused by a global cabal-I.E. monarchy. A Monarchy is nothing more than a Oligarchy.
The same as a socialist form of government.