The battle between arthur's and morded's armies starts when a soldier draws his sword to kill an adder?

Yes, you are absolutely correct. Before the battle King Arthur and Mordred decide to settle for a truce. But, before they met, each ordered their armies that if any soldier from the opposite camp draws his sWord then they should not hesitate to start the battle. Just as everything was going well, an adder slithers amidst the soldiers (I think Mordred's soldiers) and someone draws his sWord to kill it. The sWord glimmers under the sun and the battle begins.