"The assassination caused a military fight that wouldn’t have happened otherwise, but the conflicting alliances and leaders itching to use their new military power would have caused a major war anyway."

1) Europe was a powder keg. The assassination was a single item is a list of reasons a war happened.

2) The escalation of war to involve multiple countries happened only after multiple attempts at de-escalation. Saying generally that leaders wanted to go to war is a massive oversimplification of the start of the war