WASHINGTON \u2014 North Korea<\/a>\u2019s detonation of a sixth nuclear bomb on Sunday prompted the Trump administration to warn that even the threat to use such a weapon against the United States and its allies \u201cwill be met with a massive military response.\u2019\u2019<\/p>\n The test \u2014 and President Trump\u2019s response \u2014 immediately raised new questions about the president\u2019s North Korea strategy and opened a new rift with a major American ally, South Korea<\/a>, which Mr. Trump criticized for its \u201ctalk of appeasement\u201d with the North.<\/p>\n The underground blast was by far North Korea\u2019s most powerful ever. Though it was far from clear that the North had set off a hydrogen bomb, as it claimed, the explosion caused tremors that were felt in South Korea and China. Experts estimated that the blast was four to sixteen times more powerful than any the North had set off before, with far more destructive power than the bombs dropped on the Japanese cities of Hiroshima and Nagasaki during World War II<\/a>.<\/p>\n Yet after a day of meetings in the Situation Room involving Mr. Trump and his advisers, two phone calls between the president and Prime Minister Shinzo Abe of Japan<\/a>, and even demands from some liberal Democrats to cut off North Korea\u2019s energy supplies, Mr. Trump\u2019s aides conceded that they faced a familiar conundrum.<\/p>\n While the Pentagon has worked up a series of military options for targeted strikes at North Korea\u2019s nuclear and missile sites, Mr. Trump was told that there is no assurance that the United States could destroy them all in a lightning strike, according to officials with knowledge of the exchange. Cyberstrikes, which President Barack Obama ordered against the North\u2019s missile program, have also been judged ineffective.<\/p>\n Mr. Trump hinted at one extreme option: In a Twitter post just before he met his generals, he said that \u201cthe United States is considering, in addition to other options, stopping all trade with any country doing business with North Korea.\u2019\u2019<\/p>\n Taken literally, such a policy would be tantamount to demanding a stoppage of any Chinese oil to North Korea, essentially an attempt to freeze out the country this winter and bring whatever industry it has to a halt.<\/p>\n The Chinese would almost certainly balk; they have never been willing to take steps that might lead to the collapse of the North Korean regime, no matter how dangerous its behavior, for fear that South Korean and American troops would occupy the country and move directly to the Chinese border.<\/p>\n Beyond that, the economic disruption of ending all trade with China would be so huge inside the United States that Mr. Trump\u2019s aides declined on Sunday to discuss the implications.<\/p>\n After meeting with Mr. Trump, Defense Secretary Jim Mattis emerged to warn North Korea that \u201cany threat to the United States or its territory, including Guam or our allies, will be met with a massive military response.\u201d But Mr. Mattis, in a terse statement delivered on the White House driveway with the chairman of the Joint Chiefs of Staff, Gen. Joseph F. Dunford Jr., also offered a word of reassurance to the North\u2019s reclusive leader, Kim Jong-un<\/a>.<\/p>\n \u201cWe are not looking to the total annihilation of a country, namely North Korea,\u201d he said. \u201cBut as I said, we have many options to do so.\u201d<\/p>\n The statement echoed past comments by the defense secretary as well as a warning issued by President George W. Bush after North Korea\u2019s first atomic test, in 2006. In that statement, Mr. Bush also said North Korea would be held responsible if it ever exported any of its nuclear weapons<\/a> technology to other nations or to terrorists.<\/p>\n Still, Mr. Mattis\u2019s statement left open many questions. His formulation seemed to rule out the kind of \u201cpreventive war\u2019\u2019 that the national security adviser, Lt. Gen. H. R. McMaster, warned last month might be necessary after the North tested two intercontinental ballistic missiles in an effort to demonstrate that it could reach Los Angeles and beyond. Instead, Mr. Mattis seemed to be talking about \u201cpre-emptive strikes,\u201d which the United States might order if it determined that an attack seemed imminent.<\/p>\n There was no public discussion of pursuing a diplomatic opening to the North. Mr. Trump and Secretary of State Rex W. Tillerson raised such a possibility two weeks ago, after a brief lull in North Korea\u2019s testing. That statement turned out to be optimistic at best. The North has shown no interest in engaging with the United States unless the Americans end their military presence in the South.<\/p>\n To the contrary, the North Korean leader has tried to portray his nuclear program as unstoppable and nonnegotiable, posing by a picture of what the North\u2019s official news agency on Sunday called a hydrogen bomb that could be fitted into the nose cone of the ICBMs tested last month. Experts warned that the weapon, while shaped like a hydrogen bomb, could well have been a mock-up or decoy, one of the many steps the North takes to make it appear more powerful than it truly is.<\/p>\n On Monday, South Korea\u2019s army fired short-range ballistic missiles off its east coast in a simulated attack on North Korea\u2019s nuclear test site, its military said in a statement. F-15K fighter jets also joined in the show of force, firing air-to-land missiles, it said.<\/p>\n Only hours earlier, Mr. Trump reacted to the North Korean test by lashing out at South Korea.<\/p>\n \u201cNorth Korea is a rogue nation which has become a great threat and embarrassment to China, which is trying to help but with little success,\u201d he said. \u201cSouth Korea is finding, as I have told them, that their talk of appeasement with North Korea will not work, they only understand one thing!\u201d<\/p>\n Mr. Trump appeared to be referring to the offers by South Korea\u2019s new president, Moon Jae-in, to enter into some kind of negotiations with the North that might lead to a renewal of the \u201cSunshine Policy,\u201d an effort by some of his predecessors to lure the North into disarmament with economic engagement. Those efforts failed.<\/p>\n Mr. Moon said recently that he had obtained a promise from Washington that the United States would never take military action without Seoul\u2019s approval \u2014 a commitment the Trump administration has never confirmed.<\/p>\n Mr. Trump\u2019s undisguised swipe at the South for \u201cappeasement\u201d was certain to exacerbate fears that the United States might put it in danger. And it came only a day after Mr. Trump threatened a new rift in relations with suggestions that the United States might withdraw from a trade deal<\/a> with South Korea \u2014 one that was intended to bolster the alliance.<\/p>\n In response to Mr. Trump\u2019s criticism, Mr. Moon\u2019s office said it was working closely with Washington to exert \u201cmaximum sanctions and pressure.\u201d But it also reiterated that the allies shared the understanding that the goal of these sanctions and pressure was to bring North Korea back to the negotiating table.<\/p>\n \u201cWe have experienced an internecine war and can never tolerate another catastrophic war on this land,\u201d Mr. Moon\u2019s office said in a statement. \u201cWe will not give up our goal of working together with allies to seek a peaceful denuclearization of the Korean Peninsula.\u201d<\/p>\n While Washington and Seoul argue over the threat of military force, Mr. Kim seems determined to forge ahead. He has conducted more than 80 missile tests since taking over the country. And four of the six nuclear tests have been on his watch.<\/p>\n This was the biggest, by far. The United States Geological Survey estimated<\/a> that the tremor set off by the blast, detected at 12:36 p.m. at the Punggye-ri underground test site in northwestern North Korea, had a magnitude of 6.3.<\/p>\n The South Korean Defense Ministry\u2019s estimate was much lower, at 5.7, but even that would mean a blast \u201cfive to six times\u201d as powerful as the North\u2019s last nuclear test, a year ago, said Lee Mi-sun, a senior analyst at the South Korean Meteorological Administration.<\/p>\n The South\u2019s National Fire Agency, which operates an emergency hotline, said it had received 31 calls about buildings and the ground shaking, the first time that South Koreans had reported tremors after a North Korean nuclear detonation.<\/p>\n The blast was so powerful that the first tremor was followed by a second, weaker one minutes later, which the United States Geological Survey called a \u201ccollapse,\u201d probably a cave-in at the North\u2019s underground test site.<\/p>\n Condemnation of the test came from around the world.<\/p>\n China, the North\u2019s main ally and biggest trading partner, expressed \u201cstrong condemnation\u201d of the test, according to Xinhua, the state news agency, but suggested no new action. Its leaders feel as stymied as their American counterparts, according to many China experts.<\/p>\n The test\u2019s timing was a major embarrassment for President Xi Jinping of China, who on Sunday was hosting a summit meeting of the so-called BRICS countries \u2014 Brazil, Russia, India, China and South Africa.<\/p>\n Peter Hayes, director of the Nautilus Institute, a United States-based research group specializing in North Korea, said the test seemed intended to jolt Mr. Xi and convince him that he needed to persuade the United States to talk to North Korea.<\/p>\n Japan requested an emergency meeting of the United Nations Security Council, as it did earlier in the week after a missile test over Hokkaido, its northernmost island.<\/p>\n In Europe, the Russian Foreign Ministry said that North Korea \u201cdeserves absolute condemnation<\/a>,\u201d and a joint statement from Chancellor Angela Merkel of Germany and President Emmanuel Macron of France said \u201cthe most recent provocation from Pyongyang reaches a new dimension.\u201d<\/p>\n The International Atomic Energy Agency<\/a> said the test amounted to a \u201ccomplete disregard of the repeated demands of the international community.\u201d<\/p>\n The timing of the test on Sunday was almost certainly no coincidence: It came during the American Labor Day weekend, and the anniversary of the founding of the North Korean government is next Saturday.<\/p>\n In the coming days, the government is expected to organize huge rallies to celebrate the bomb test and Mr. Kim\u2019s leadership.<\/p>\n \u201cPyongyang has a playbook of strategic provocations, throws off its adversaries through graduated escalation, and seeks maximum political impact by conducting weapons tests on major holidays,\u201d said Lee Sung-yoon, a Korea expert at the Fletcher School of Law and Diplomacy at Tufts University.<\/p>\n