A North Korean ability to reach the United States, as former Defense Secretary William J. Perry noted recently, “changes every calculus.” The fear is not that Mr. Kim would launch a pre-emptive attack on the West Coast; that would be suicidal, and if the North’s 33-year-old leader has demonstrated anything in his five years in office, he is all about survival. But if Mr. Kim has the potential ability to strike back, it will shape every decision Mr. Trump and his successors make about defending America’s allies in the region.

For years, the North’s medium-range missiles have been able to reach South Korea and Japan with ease, and American intelligence officials believe the missiles are capable of carrying nuclear warheads.

But this latest test suggests that the United States may already be in range as well, and that, as one former top American intelligence official noted recently, would put enormous pressure on American missile defenses that few trust to work.