Almost everything American intelligence agencies and North Korea-watchers thought they understood two years ago about Kim Jong-un, the North's young leader, turns out to have been wrong.
The briefings given to President Obama after Mr. Kim inherited leadership said it was almost certain he would be kept in check by his more experienced uncle, Jang Song-thaek. Instead, Mr. Kim had his uncle and dozens of others executed.
The early betting was also that Mr. Kim, who was briefly educated in Switzerland, would emphasize economic overhaul over expanding the nuclear and missile arsenals that were his father's and grandfather's legacy. Instead, the nuclear program has surged forward, and recent missile tests are demonstrating that after years of spectacular failures, the North's engineers are finally improving their aim. Their next big challenge is proving that an intercontinental missile they have shown only in mock-ups can reach America's shores.
As a result, when Mr. Obama lands here on Friday on the second stop of his Asia tour, he will be confronting the question of whether his strategy of "strategic patience" with the North has been overtaken by reality: an unpredictable, though calculating, ruler in Mr. Kim, who has proved to be more ruthless, aggressive and tactically skilled than anyone expected.
"We have failed," said Evans J. R. Revere, who spent his State Department career trying various diplomatic strategies to stop the North. "For two decades our policy has been to keep the North Koreans from developing nuclear weapons. It's now clear there is no way they will give them up, no matter what sanctions we impose, no matter what we offer. So now what?"
It is an assessment some of Mr. Obama's aides say they privately share, though for now the administration refuses to negotiate with the North until it first fulfills its oft-violated agreements to freeze its nuclear and missile programs. A recent effort inside the National Security Council to devise a new approach resulted in a flurry of papers and classified strategy sessions — and the conclusion that all the alternatives to the current course were worse.
"We're stuck," one participant in the review said.
The only place any real change is visible is in the military planning by South Korea and the United States, which maintains a shrunken force of 28,000 troops in the South. For the first time since the armistice in 1953, officials say, the contingency plan for a conflict with the North treats the nation as a nuclear-capable adversary, despite the administration's official refusal to acknowledge it as a de facto nuclear state. (What appear to be North Korea's preparations for a fourth nuclear test, perhaps in the coming days, seem intended to remove all doubt.)
The latest revision of OpPlan 5029, the war plan for the Korean Peninsula, assumes that if a conflict broke out, the North would be able to deliver a crude nuclear weapon, though perhaps by truck or ship. American intelligence officials do not believe the North is yet able to shrink a bomb to a size that could fit on one of its Nodong missiles, the key breakthrough it needs.
"He's put new effort into his nuclear program, missiles, special operations forces and long-range artillery," said Gen. Curtis M. Scaparrotti, who took over last fall as commander of United States Forces Korea and the United Nations Command here. "They are using more underground facilities. He's gone to school on how we operate."
Defense officials say they now have less warning time on missile launchings than they had two or three years ago because Mr. Kim has put his resources into mobile launchers that are regularly moved from tunnel to tunnel, making them harder for American satellites to track.
Although details of the revised plan are classified, officials have talked about elements of it. Since the North shelled a South Korean island and was blamed for sinking a South Korean warship four years ago, there are now extensive plans for immediately responding to and then de-escalating small attacks along the border regions.
The North Korean forces remain numerically impressive at a million soldiers, but highly unimpressive when they train. The country is so poor that South Korean military officials say its pilots rarely have the gas to perform practice runs.
But in recent interviews here and in Washington, a picture has emerged of Mr. Kim's new focus on inexpensive weaponry, from missile launchers to crude cyberweapons, that are hard to detect and harder to halt. Mr. Kim, who is believed to be 30 years old, has also nurtured his reputation for unpredictability, keeping adversaries on edge.
Administration officials acknowledge they have largely left North Korea on the back burner while focusing on sanctions, cyberattacks and pressure on Iran, forcing it into negotiations.
"The administration decided, consciously or implicitly, that Iran was more important and there was a greater prospect of getting something done," said Robert Einhorn, who ran the sanctions enforcement program against both countries until he left the State Department last year. "While you can squeeze Iran and its oil money, it's much harder to squeeze North Korea" while China continues its financial support.
White House officials argue that focusing first on Iran made sense. Its program can still be halted before it gains a weapons ability, if that is Tehran's goal, and the administration believes that North Korea is less likely to set off a regional arms race.
"You could argue that the best North Korea strategy now is to get a deal with Iran, and use it as a model for the North about what the world can look like," one senior administration official said.
But others inside the administration fear that policy is too passive — and perhaps a prescription for a much larger North Korean arsenal by the time Mr. Obama leaves office.
At the heart of the problem are dashed hopes that Mr. Kim would conclude that his grandfather's and father's pursuit of a nuclear ability was a Cold War relic, and that he would gradually steer the country to integration with the world economy. There was modest reason for optimism just months after Mr. Kim came to power in 2011 and struck yet another deal to freeze all his nuclear and missile activity, in return for a resumption of the episodic six-party talks with the United States and other nations. That brief effort ended when the North launched a satellite in honor of Mr. Kim's grandfather. Diplomacy froze for the next two years, with the administration unwilling to make concessions as previous administrations did only to find that the North was reneging on its promises.
In recent months the Chinese have led an effort to restart diplomatic talks, and the United States has quietly met with the North. But the goal is unclear. To the United States, the purpose of the talks would be denuclearization; Mr. Kim's government has already declared that the one thing he will not do is give up his small nuclear arsenal, especially after seeing the United States help unseat Col. Muammar el-Qaddafi of Libya, who surrendered his own nuclear program in 2003.
Joel Wit, a former North Korea strategist for the American government, said Mr. Kim drew an indelible lesson from that history. "It's not an accident he's positioning himself to make sure the inventory of nuclear material in the hands of the North is about to take off," said Mr. Wit, who edits 38 North, a website that follows the murky, often murderous politics of the Kim government.
He was referring to the North's effort to expand the production of highly enriched uranium, which would give Mr. Kim a steadier, more plentiful supply of nuclear fuel than its past reliance on extracting plutonium from a small nuclear reactor.
"I'm now convinced North Korea would prefer to collapse with nuclear weapons than try to survive without nuclear weapons," Chun Yung-woo, who recently served as the South's national security adviser, said this week. Yet the strategy Washington is pursuing is based on the opposite assumption.