I would say alliances and nationalism because nationalism only encouraged countries to go to war and prove that they were the best most powerful country around and in terms of WWI, nationalism led to strong dedication to the army and just keep most countries in the war longer than they should have been in it. Alliances were significant in the exact beginning of the war and the scale to which the war was fought. The many (secret) alliances led to austria-hungary and Serbia to sorta threaten each other, which brought Germany in, then France, etc.
