October 10 is the anniversary of the NK communist party. Missile movements have been spotted, NK usually celebrates such holidays with some military demonstration, and NK has threatened an H-bomb test in the Pacific --- which would mean not just lobbing missiles over Japan again and making people run for cover again, but lobbing an H-bomb over Japan.
I think that's probably what Trump was talking about --- perhaps taking proactive measures against an expected upcoming missile launch/H-bomb test in the Pacific --- which NK will probably construe as an act of war.