North Korea's sixth nuclear test – coming hours after state media showed Kim Jong Un inspecting a new thermonuclear warhead for an intercontinental ballistic missile (ICBM) – adds additional urgency to an already critical issue for the U.S. and its allies.

Following two ICBM tests in July and two successful intermediate range ballistic missile (IRBM) tests in May and August, Pyongyang is making rapid progress on the final stages of testing of weapons systems capable of delivering high yield nuclear explosives to cities across the United States.