John Kerry, less than two weeks into his new job, has joined the long list of U.S. secretaries of states who expressed firm resolve to deter North Korea from advancing its nuclear weapons program.

But such determination was expressed before — and after — the Stalinist regime first obtained a nuclear weapon more than six years ago.

Thus, the tyrants who run North Korea have heard it all — and defied it all — many times before.

And while the Kim dynasty, now headed by Kim Jong Un, has provided the outside world some dark amusement over the decades, its comic relief has dwindled as its nuclear arsenal has grown.

And North Korea hasn’t only made frightful progress on nuclear weapons. It has advanced its missile program, too.

North Korea recently conducted an underground test of its smallest nuclear weapon yet. The smaller those weapons are, the closer North Korea gets to producing one that could be carried by a missile.

On the missile front, after a series of embarrassing failures, North Korea finally succeeded two months ago in launching a rocket that soared over Okinawa and dropped debris into the East China Sea and waters near the Philippines.

Ponder the chilling combination of a nuclear warhead that would fit on a missile and a missile that can travel long distances. Ponder North Korea’s improvement in controlling the course of such a missile.

Ponder the distance from North Korea to South Korea, Japan — and Alaska, or even the West Coast.

At least Secretary Kerry said the right things last week about North Korea’s latest nuclear arms test.

He called for international teamwork on a “swift, clear, strong and credible response.” He stressed that mere words from the global community would not suffice: “If you are going to say things, they have to mean something. And to mean something you have to be prepared to follow up, and that’s exactly what we are prepared to do.”

The familiar and intensifying problem, though, is that past talk — and past economic sanctions — haven’t dissuaded North Korea from its reckless policies. The regime has long poured considerable resources into weaponry while many of its people starve.

Meanwhile, despite America’s repeated appeals for China to bring North Korea into line, it has yet to do so.

At least China did condemn last week’s nuclear weapons test. But China has scolded North Korea before, even as that rogue nation keeps striving toward ominous nuclear options.

At this alarming point, however, the U.S. must again try to re-focus not just China but the rest of the world on the increasingly urgent need to deter North Korea’s martial — and nuclear — ambitions.

That means stricter sanctions, realistic negotiations and renewed vigilance in forging international unity on this high-priority challenge.

And judging from the long-term futility of numerous secretary of state predecessors on this harrowing menace, Mr. Kerry should know that “to follow up” on tough talk against North Korea remains a daunting task.