SEOUL—U.S. and South Korean military leaders this year developed broad new countermeasures and plans for dealing with North Korean attacks against the South, amid pressure in Seoul to take a more aggressive stance toward its rival in the future, the commanding general of the combined forces said.

U.S. Gen. Walter Sharp said in an interview Thursday, ahead of his retirement next week, that the process emerged from routine high-level dialogues between the two allies that picked up pace and intensity after Pyongyang's attacks...