David A. Andelman, visiting scholar at the Center on National Security at Fordham Law School and director of its Red Lines Project, is a contributor to CNN and columnist for USA Today. Author of " A Shattered Peace: Versailles 1919 and the Price We Pay Today ," he formerly served as a foreign correspondent for The New York Times and CBS News in Asia and Europe. Follow him on Twitter @DavidAndelman . The views expressed in this commentary are his.

A senior South Korean delegation had just spent the afternoon with the President and his national security adviser, H.R. McMaster, carrying a remarkable letter from North Korean dictator Kim Jong Un.

It was an invitation by Kim to meet with the President as soon as possible. And Trump promptly accepted. He had no choice -- if he hadn't, the world would be poised for a long, potentially deadly spiral into nuclear conflict. But there is still a tortuous path ahead before we'll see an end to the nuclear threat on the Korean Peninsula and far beyond.

Acceptance of Kim's invitation means reversing a long string of threats and calumny Trump has heaped on the North Korean leader -- all of which he appeared to believe in devoutly until now. And that will require some remarkable and utterly uncharacteristic restraint.

No longer will Trump be able to call the North Korean leader " little Rocket Man " as he did from the podium of the United Nations General Assembly last September; no longer will he be able to threaten the total destruction of North Korea if its ruler trains its weapons on any American facility or ally.

Not surprisingly, however, the devil is in the details. The letter from Kim Jong Un carried a simple offer -- to halt nuclear and missile tests before the meeting of the two leaders, which could take place as early as May. But we are far from North Korea taking measurable steps toward dismantling its the nuclear weapons program, or from the denuclearization of the Korean Peninsula.

We must also understand the motives, especially, of the North Korean side, and the reservations that could still linger in the Trump White House. Was this letter an attempt to take the bootheel of increasingly onerous sanctions off North Korea's throat? Are the sanctions really having a far greater impact than is apparent from the outside?

If so, what might be won by waiting six months, even a year? Clearly, North Korea's offer to meet "as soon as possible" suggests an urgency and hints at other motives for Kim, who only recently, was boasting of his ability to target the entire mainland United States with a nuclear armed missile.

Could the offer equally be part of a shrewd effort to separate the United States from South Korea and other American allies, many of them already quite skeptical about Trump's bluster? If the peace process should collapse, fingers of blame might be more readily pointed at Washington than Pyongyang -- and the likelihood of further sanctions eased.

Perhaps Kim is confident that he has gained all the nuclear knowledge he needs, believing he has assembled a workable rocket and nuclear warhead. Now is the moment to buy some time to swing into production so that an arsenal can be assembled, giving him more nuclear might to trade off than he has now.

Above all, Kim may also believe that it is time to prove himself a world leader on a par with Trump. A summit of equals could go far in such a direction. All this may play into Trump's embrace of the entire project.

Still, this is a unique, even historic opportunity. And Trump does love historic firsts. No meeting has ever taken place between an American president and either of the three generations of Kims who have ruled North Korea since the end of the Korean War more than six decades ago.

And the timing is quite remarkable. First, there is a new, liberal and peace-minded government in South Korea anxious to defuse a situation that threatened to wipe its nation from the map in any conflict on the peninsula it shares with the north. Second, there is the increasing tension on a number of fronts between the United States and China. The Chinese leadership would certainly prefer to see Pyongyang find a way to remove pressure from Washington than to ratchet up further sanctions on its North Korean ally.

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Even so, a complete denuclearization of the Korean Peninsula is quite simply not going to happen -- certainly not by May. This must be taken in baby steps. In the 1980s, it took more than two years of negotiations between senior diplomats of the United States and the Soviet Union before a summit between Ronald Reagan and Mikhail Gorbachev was even possible -- talks that I followed from beginning to end. And that summit was simply designed to consider an arms control treaty , not the complete dismantling of both sides' nuclear arsenals -- a proposal made by Gorbachev and promptly rejected Reagan.

This is not to say that there are not other intermediate steps that are entirely possible. And they must be explored.

But dramatic gestures of this type require enormous preparation and deep wells of goodwill and restraint by all parties -- virtues that neither side has shown itself capable of mastering. Perhaps this is a first, and critical test -- one that Trump must pass at all cost.