The presence of U.S. armed forces in South Korea has demonstrated America’s commitment to that nation’s defense for more than six decades. But on Friday America sent a fresh — and timely — signal of that resolve as U.S. Secretary of State John Kerry arrived in Seoul.

Secretary Kerry served this timely notice from President Barack Obama: “The United States will, if needed, defend our allies and defend ourselves.”

Over the last few weeks, the Obama administration has provided numerous such reminders regarding potential North Korean aggression against not just South Korea but Japan.

That’s a critical element in responding to North Korean dictator Kim Jung Un’s bellicose rhetoric and reckless actions.

So is America’s continuing effort to convince China that it should, in its own interests, use its considerable leverage over North Korea to bring Kim back into line.

Secretary Kerry will press that case anew when he visits Beijing before making Tokyo the last stop on his four-day East Asia trip.

North Korea appears prepared to launch a medium-range missile test, perhaps as soon as today on the anniversary of the birthday of Kim’s grandfather, Kim Il Sung, the founder of the dynastic, Stalinist regime.

Secretary Kerry fairly warned:

“If Kim Jong Un decides to launch a missile, whether it’s across the Sea of Japan or in some other direction, he will be choosing willfully to ignore the entire international community, his own obligations that he has accepted, and it will be a provocative and unwanted act that will raise people’s temperature with respect to this issue.”

And as Secretary Kerry put it Friday, if war breaks out, “Kim Jong Un needs to understand, as I think he probably does, what the outcome of the conflict would be.”

A Pentagon intelligence report released this week concluded, with “moderate confidence,” that North Korea has achieved the technical expertise to launch a missile with a nuclear warhead.

But U.S. military officials quickly disputed that finding.

And Secretary Kerry said Friday in Seoul:

“We do not operate under the assumption that they have that fully tested and available capacity.”

He added, however: “They have conducted a nuclear test, so there’s some kind of device. That is very different from miniaturization and delivery. Does it get you closer to the line that is more dangerous? Yes.”

North Korea has clearly moved more closely to a very dangerous line over the last month. So it was reassuring to see Secretary Kerry take his stand in South Korea.

And it was downright gratifying to hear him point out who the biggest loser would be if Kim starts a war.