But just as the Defense Department seeks to prove that it can strike a speeding target launched over the Pacific — in this case, an interceptor rocket is set to lift off from the California coast on Tuesday to try to smash a mock warhead — the North Koreans have delivered a new challenge.

The North has recently test-fired a series of missiles based on a technology that would give the United States little warning of an attack. The new generation of missiles uses solid fuels, enabling them to be rolled out from mountain hideaways and launched in minutes. That makes the job of intercepting them — already daunting — far harder, given that the American antimissile system works best with early alerts from satellites that a launch is imminent.

Even more worrisome is that these missiles actually seem to be functional, unlike older missiles that kept exploding or falling prematurely into the sea in past tests. Recent major tests were clearly successful, teaching the North Koreans a lot about how to fire missiles into space and drop warheads on distant targets. While the North has not yet flight-tested an intercontinental ballistic missile capable of crossing the Pacific, it has repeatedly claimed that it can strike the United States with a nuclear warhead.