It would seem a sub attack is in order.
According to OPLAN-5027,the plan previously supposed a possible invasion by North Korea. However, the plan adopted the strategies of “pre-emptive attacks.” If “conclusive evidence” suggesting North Korea’s war preparation is detected, the plan allows for a pre-emptive attack of the ROK and the U.S. alliance. The US and ROK can consider this sub attack a “pre-emptive” attack by North Korea.