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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.