When King Orm (Patrick Wilson) goes to meet King Nereus (Dolph Lundgren) for the first time, they get attacked by a submarine.
The submarine shoots torpedoes at them without provocation. It is then sunk by Orm in retaliation. This event convinces Nereus that the surface world must be attacked now. Till this point he was on the fence about initiating a war. This event makes it plain that the surface world has already begun their offensive and must be answered.
However, it seems like people on the surface world aren't aware of the existence of Atlantis (or sentient sea creatures for that matter). It is something which seems to be treated as an urban legend based on the news bit involving Randall Park. This raises questions on the authenticity of the submarine attack. Was it genuinely an attack by the surface world or something staged by Orm to sway the tide in his favor?