Suspected North Korean hackers stole sensitive military secrets, including a joint U.S.-South Korean plan detailing how to eliminate the Pyongyang leadership in the event of war, in a breach of Seoul’s defense data system last year, U.S. defense officials and a South Korean lawmaker said.

The hackers broke into South Korea’s defense database in September last year and snatched a trove of the classified military documents, Rhee Cheol-hee, a member of the ruling Democratic Party, said in an interview with local media published...