Timeline of the North Korean nuclear program

Phase II (1980–94) covers the growth and eventual suspension of North Korea's domestic plutonium production program.

Phase III (1994–2002) covers the period of the "freeze" on North Korea's plutonium program (though North Korea pursued uranium enrichment in secret)[3] and Phase IV (2002–present) covers the current period of renewed nuclear activities.

Compared to fission weapons, thermonuclear designs are exceedingly complex, and staged weapons in particular are so complex that only five countries (USA, Russia, France, UK, China) have created them in more than 70 years of research.

[5] Several countries with long-standing nuclear weapons programs, such as India and Pakistan, are suspected of striving towards a hybrid or "boosted" design instead, which is easier.

[5] In 2018, North Korea had offered and was reportedly preparing for inspections at nuclear and missile sites.