North Korea attacked a tiny South Ko rean island yesterday, killing at least two marines in a shower of wholly unprovoked artillery fire that also injured at least 16 soldiers and civilians.

Seoul promised to retaliate vigorously in the event of another attack — which is both its right and duty.

The attack follows the torpedoing of a South Korean frigate in March.

The first successful submarine attack on a combat vessel since the Falklands War, it killed 46 sailors.

How much, in other words, can Seoul reasonably be expected to take?

And what should Washington do?

“It’s hard to pile more sanctions upon the North than are already there,” said Pentagon spokesman Geoff Morrell.

And he’s right: It’s hard to design effective sanctions against a regime that starves its people as a matter of policy.

In any event, North Korea is building a new nuke reactor and reportedly has 2,000 high-tech centrifuges enriching uranium — news that shocked US intelligence last week.

But why was anybody surprised?

North Korea has been spreading its nuclear know-how to the most dangerous corners of the globe, selling technology to Iran, Syria and Burma, according to a bombshell UN report out this month.

Mark it as another dismal result of a carrot-and-stick nuclear-containment policy that’s failed utterly — across three presidencies — for the last 15 years.

So now it’s President Obama’s turn to tread water — hoping to contain the North without setting off more attacks or

cutting a shallow deal like his predecessors.

In this respect, he is not alone.

South Korea and Japan stand equally impotent in the face of Pyongyang’s challenges. And China, which reputedly has some influence with the North, appears unwilling to exercise it.

That may well be because the only realistic long-term policy — regime change, preferably peacefully accomplished — likely would leave China sharing a border with a reunified, US-allied, South Korea.

And Beijing made it quite clear what it thinks of that prospect in 1950.

But China won’t benefit from a catastrophic collapse of North Korea either — especially if it’s accompanied by a nuclear tantrum of one sort or another.

It would be useful for the White House to exercise whatever influence it still may have with Beijing to effect a reasonably peaceful solution — that is, if it is even capable of such nuanced maneuvering.

Certainly, time’s a-wasting.

North Korea has more than 10,000 artillery tubes zeroed in on Seoul — and its next attack might well be on a bigger target than an isolated off-shore island.

Even a relatively small-scale exchange of fire across the 38th Parallel would severely damage the South’s economy — with global ramifications.

Obama & Co. don’t seem to take much seriously when it comes to international relations.

But a mistake here could do incalculable damage.