To be fair the Casus Belli was already in play by time of the Burning of Teldrassil. I mean he sent spies into Silithus as reconnaissance and with orders to hinder the Horde's actions in the area as preparatory measures for an assault before shifting most of the Night Elven army there after believing the Horde were going to set up there first which Saurfang exploited for the attack on Darnassus. He then immediately responded to that by sending most of Stormwind's forces north for a beach assault on Lordaeron and a siege he would have lost if not for Jaina literally popping up out of nowhere. We could also consider his agreement of letting Calia attend the Forsaken Gathering and attempting to get people to defect as another contribution to the war tbh.