WarSclerotic

GENEVA — North Korea’s latest test of an atomic weapon leaves the United States with an uncomfortable choice: stick with a policy of incremental sanctions that has clearly failed to stop the country’s nuclear advances, or pick among alternatives that range from the highly risky to the repugnant.

A hard embargo, in which Washington and its allies block all shipping into and out of North Korea and seek to paralyze its finances, risks confrontations that allies in Asia fear could quickly escalate into war. But restarting talks on the North’s terms would reward the defiance of its young leader, Kim Jong-un, with no guarantee that he will dismantle the nuclear program irrevocably.

For more than seven years, President Obama has sought to find a middle ground, adopting a policy of gradually escalating sanctions that the White House once called “strategic patience.” But the test on Friday — the North’s fifth and most powerful blast yet, perhaps with nearly twice the strength of its last one — eliminates any doubt that that approach has failed and that the North has mastered the basics of detonating a nuclear weapon.

Despite sanctions and technological backwardness, North Korea appears to have enjoyed a burst of progress in its missile program over the last decade, with experts warning that it is speeding toward a day when it will be able to threaten the West Coast of the United States and perhaps the entire country.

“This is not a cry for negotiations,” said Victor Cha, who served in the administration of President George W. Bush and now is a North Korea expert at the Center for Strategic and International Studies in Washington. “This is very clearly a serious effort at amassing real nuclear capabilities that they can use to deter the U.S. and others.”

Mr. Cha said the usual response from Washington, Seoul and Tokyo — for another round of sanctions — was not likely to be any more successful at changing the North’s behavior than previous rounds. That means Mr. Obama’s successor will confront a nuclear and missile program far more advanced than the one Mr. Obama began grappling with in 2009.

Mr. Obama has refused to negotiate with the North unless it agrees first that the ultimate objective of any talks would be a Korean Peninsula without nuclear arms. But Mr. Kim has demonstrated, at least for now, that time is on his side. And as he gets closer to an ability to threaten the United States with a nuclear attack, and stakes the credibility of his government on it, it may be even more difficult to persuade him to give up the program.

“To be clear, the United States does not, and never will, accept North Korea as a nuclear state,” he said.

Many experts who have dealt with North Korea say the United States may have no choice but to do so.

“It’s too late on the nuclear weapons program — that is not going to be reversed,” William Perry, the defense secretary under President Bill Clinton during the 1994 nuclear crisis with North Korea, said in August at a presentation in Kent, Conn. The only choice now, he argued, is to focus on limiting the missile program.

Yet the latest effort to do that, an agreement between the United States and South Korea to deploy an advanced missile defense system in the South, has inflamed China, which argues the system is also aimed at its weapons. While American officials deny that, the issue has divided Washington and Beijing so sharply that it will be even more difficult now for them to come up with a joint strategy for dealing with the North.

China has been so vocal with its displeasure over the deployment of the American system that Mr. Kim may have concluded he could afford to upset Beijing by conducting Friday’s test.

Fueling that perception were reports that a North Korean envoy visited Beijing earlier this week.

“North Korea almost certainly sees this as an opportunity to take steps to enhance its nuclear and missile capabilities with little risk that China will do anything in response,” Evans J.R. Revere, a former State Department official and North Korea specialist, said in a speech in Seoul on Friday.

The breach between China and the United States was evident during Mr. Obama’s meeting with President Xi Jinping last week. “I indicated to him that if the Thaad bothered him, particularly since it has no purpose other than defensive and does not change the strategic balance between the United States and China, that they need to work with us more effectively to change Pyongyang’s behavior,” Mr. Obama said, referring to the Terminal High Altitude Area Defense system, as the advanced missile defense project is known.

But Mr. Obama noted that sanctions had failed at having much effect. That is largely because the Chinese have left open large loopholes that have kept the North Korean economy alive and, by some measures, enjoying more trade than anytime in years.

In a recent paper, two researchers concluded that sanctions so far “have had the net effect of actually improving” North Korea’s procurement capabilities for its weapons program. To evade sanctions, the North’s state-run trading companies opened offices in China, hired more capable Chinese middlemen and paid higher fees to employ more sophisticated brokers, according to Jim Walsh and John Park, scholars at M.I.T. and Harvard respectively.

The sanctions, Mr. Cha noted, “are supposed to inflict enough pain so the regime comes back to the negotiation table, and that’s clearly not working; or it’s supposed to collapse the regime until it starves, and that’s not working either.”

“Unless China is willing to cut off everything, which they don’t appear willing to do, the sanctions may be politically the right thing to do and a requisite response, but they are not the answer to the problem,” he said.

That means the choices facing Mr. Obama’s successor will be stark. One option is to choke off all trade, in part by telling banks that conduct transactions with North Korea that they will be shut out of dealing in dollars around the world — an effective tactic against Iran before last year’s nuclear deal. But that would enrage the Chinese, and probably cut into cooperation on other issues.

At the same time, an attempt to intercept all shipping could quickly escalate into a full-blown conflict, something neither Mr. Obama nor the South Koreans and Japanese have been willing to risk.

On the other hand, reopening negotiations, which Donald J. Trump has indicated he is willing to consider, could mean paying North Korea again to freeze nuclear activities that the Bush administration and the Clinton administration had already rewarded them for stopping years ago.

The nuclear program dates back to Mr. Kim’s grandfather, Kim Il-sung, the country’s founder, who emerged from the Korean War more than 60 years ago mindful that the United States had considered using nuclear weapons in that conflict and determined to get his own arsenal.

The missile program also has a long history, mostly to deliver conventional arms. But now the two are converging, as the North races to develop a weapon small, light and durable enough to be launched into space and survive re-entry into the atmosphere.

The explosive energy unleashed during the test on Friday, estimated at 10 to 12 kilotons of TNT, was nearly twice that of the North’s last test, conducted in January, said Yoo Yong-gyu, a senior seismologist at South Korea’s National Meteorological Administration.

And the fact that North Korea’s fifth test came only eight months after its fourth is another indication that it is making fast progress toward fitting its ballistic missiles with nuclear warheads, said Choi Kang, a senior analyst at the Asan Institute. The North had waited about three years between each of its previous tests.

North Korea’s advances have unnerved its neighbors in South Korea and Japan, and Mr. Trump’s suggestion that the two nations should pay more for the United States to defend them has not helped.

In both South Korea and Japan, a small but increasingly vocal minority hasbegun to advocate developing nuclear weapons to counter the North instead of relying on the United States.

Cheong Seong-chang, a senior analyst at the Sejong Institute in Seongnam, south of Seoul, argued that a South Korean nuclear program might distract the North from its efforts to build a long-range missile capable of delivering a nuclear warhead to the mainland United States.

“If South Korea arms itself with nuclear weapons, North Korea will regard the South Korean nuclear weapons, not the distant American nukes, as the most direct threat to its security,” Mr. Cheong said.