Concerns are growing that North Korea will mark this week’s 71st anniversary of the founding of its Workers’ Party with fireworks. After five nuclear tests in a decade, the biggest and most recent in August, a sixth may be in the offing: satellite pictures show increased activity near the tunnels of the Punggye-ri nuclear test site. South Korea is maintaining a “quasi-emergency posture”. Whether a device is exploded or not, missile tests will surely continue—with nearly two dozen this year. Kim Jong Un’s regime seems to be making rapid progress in attempts to combine a nuclear warhead and targeting technology with a missile capable of striking mainland America; it may be only a few years away. What can America do about it? Pre-emptive strikes, though talked about, would be crazy, given cosmopolitan Seoul’s vulnerability to massive retaliation. But financial sanctions on Mr Kim’s odious regime could be tightened much further.