One by one, the United States has hit at the core tenets of Xi Jinping’s vision for a rising China ready to assume the mantle of superpower.In a matter of weeks, the Trump administration has imposed sanctions over punitive policies in Hong Kong and China’s western region of Xinjiang. It took new measures to suffocate Chinese innovation by cutting it off from American technology and pushing allies to look elsewhere. On Monday, it challenged China’s claims in the South China Sea, setting the stage for sharper confrontation.And President Trump said on Tuesday that he had signed into law a bill to punish Chinese officials for the new security law that curbs the rights of Hong Kong residents, along with an executive order ending preferential trade treatment for Hong Kong.“The power gap is closing, and the ideological gap is widening,” said Rush Doshi, director of the China Strategy Initiative at the Brookings Institution in Washington, adding that China and the United States had entered a downward “ideological spiral” years in the making.“Where’s the bottom?” he asked.For years, officials and historians have dismissed the idea that a new Cold War was emerging between the United States and China. The contours of today’s world, the argument went, are simply incomparable to the decades when the United States and the Soviet Union squared off in an existential struggle for supremacy. The world was said to be too interconnected to easily divide into ideological blocs.Now, lines are being drawn and relations are in free fall, laying the foundation for a confrontation that will have many of the characteristics of the Cold War — and the dangers. As the two superpowers clash over technology, territory and clout, they face the same risk of small disputes escalating into military conflict.The relationship is increasingly imbued with deep distrust and animosity, as well as the fraught tensions that come with two powers jockeying for primacy, especially in areas where their interests collide: in cyberspace and outer space, in the Taiwan Strait and the South China Sea, and even in the Persian Gulf. And the coronavirus pandemic, coupled with China’s recent aggressive actions on its borders — from the Pacific to the Himalayas — has turned existing fissures into chasms that could be difficult to overcome, no matter the outcome of this year’s American presidential election.From Beijing’s perspective, it is the United States that has plunged relations to what China’s foreign minister, Wang Yi, said last week was their lowest point since the countries re-established diplomatic relations in 1979.“The current China policy of the United States is based on ill-informed strategic miscalculation and is fraught with emotions and whims and McCarthyist bigotry,” Mr. Wang said, evoking the Cold War himself to describe the current level of tensions.“It seems as if every Chinese investment is politically driven, every Chinese student is a spy and every cooperation initiative is a scheme with a hidden agenda,” he added.Domestic politics in both countries have hardened views and given ammunition to hawks.“What cooperation is there between China and the United States right now?” said Zheng Yongnian, director of the East Asian Institute at the National University of Singapore. “I can’t see any substantial cooperation.”The pandemic, too, has inflamed tensions, especially in the United States. Mr. Trump refers to the coronavirus with racist tropes, while Beijing accuses his administration of attacking China to detract from its failures to contain the virus.Mr. Trump, in a statement delivered from the Rose Garden Tuesday evening that focused harshly on China and his presidential rival, Joseph R. Biden Jr., referred to the pandemic as “the plague pouring in from China,” and said that the Chinese “could have stopped it.”Both countries are forcing other nations to take sides, even if they are disinclined to do so. The Trump administration, for example, has pressed allies — with some success in Australia and, on Tuesday, in Britain — to forswear the Chinese tech giant Huawei as they develop 5G networks. China, facing condemnation over its policies in Xinjiang and Hong Kong, has rallied countries to make public demonstrations of support for them.At the United Nations Humans Rights Council in Geneva, 53 nations — from Belarus to Zimbabwe — signed a statement supporting China’s new security law for Hong Kong. Only 27 nations on the council criticized it, most of them European democracies, along with Japan, Australia and New Zealand. Such blocs would not have been unfamiliar at the height of the Cold War.China has also wielded its vast economic power as a tool of political coercion, cutting off imports of beef and barley from Australia because its government called for an international investigation into the origins of the pandemic. On Tuesday, Beijing said it would sanction the American aerospace manufacturer Lockheed Martin over recent weapons sales to Taiwan.With the world distracted by the pandemic, China has also wielded its military might, as it did by testing its disputed frontier with India in April and May. That led to the first deadly clash there since 1975. The damage to the relationship could take years to repair.Increasingly, China seems willing to accept the risks of such actions. Only weeks later, it asserted a new territorial claim in Bhutan, the mountain kingdom that is closely allied with India.With China menacing vessels from Vietnam, Malaysia and Indonesia in the South China Sea, the United States dispatched two aircraft carriers through the waters last month in an aggressive show of strength. Further brinkmanship appears inevitable now that the State Department has declared China’s claims there illegal.A spokesman for China’s foreign ministry, Zhao Lijian, said on Tuesday that the American declaration would undermine regional peace and stability, asserting that China had controlled the islands in the sea “for thousands of years,” which is not true. As he stated, the Republic of China — then controlled by the Nationalist forces of Chiang Kai-shek — only made a formal claim in 1948.“China is committed to resolving territorial and jurisdictional disputes with directly related sovereign states through negotiations and consultations,” he said.That is not how its neighbors see things. Japan warned this week that China was attempting to “alter the status quo in the East China Sea and the South China Sea.” It called China a more serious long-term threat than a nuclear-armed North Korea.Michael A. McFaul, a former American ambassador to Russia and professor of international studies at Stanford University, said China’s recent maneuvering appeared to be “overextended and overreaching,” likening it to one of the most fraught moments of the Cold War.“It does remind me of Khrushchev,” he said. “He’s lashing out, and suddenly he’s in a Cuban missile crisis with the U.S.”A backlash against Beijing appears to be growing. The tensions are particularly clear in tech, where China has sought to compete with the world in cutting-edge technologies like artificial intelligence and microchips, while harshly restricting what people can read, watch or listen to inside the country.If the Berlin Wall was the physical symbol of the first Cold War, the Great Firewall could well be the virtual symbol of the new one.What began as a divide in cyberspace to insulate Chinese citizens from views not authorized by the Communist Party has now proved to be a prescient indicator of the deeper fissures between China and much of the Western world.Mr. Wang, in his speech, said China had never sought to impose its way on other countries. But it has done exactly that by getting Zoom to censor talks that were being held in the United States and by launching cyberattacks on Uighurs across the globe.Its controls have been hugely successful at home in stifling dissent and helping to seed domestic internet giants, but they have won China little influence abroad. India’s move to block 59 Chinese apps threatens to hobble China’s biggest overseas internet success to date, the meme-laden short-video app TikTok.Last week, TikTok also shut down in Hong Kong because of China’s new national security law there. The American tech giants Facebook, Google and Twitter said they would stop reviewing data requests from the Hong Kong authorities as they assessed the law’s restrictions.“China is big, it will be successful, it will develop its own tech, but there are limits to what it can do,” said James A. Lewis, a former American official who writes on cybersecurity and espionage for the Center for Strategic and International Studies in Washington. Even in places where China has succeeded in selling its technology, the tide appears to be turning.Beijing’s recent truculence has now led the United Kingdom to block new Huawei equipment from going into its networks, and the Trump administration is determined to cut the company off from microchips and other components it needs. To counter, Beijing has redoubled efforts to build homegrown options.Calls for a total decoupling of China’s supply chain from American tech companies are unrealistic in the short term, and would prove massively expensive in the longer term. Still, the United States has moved to pull Taiwan’s microchip manufacturing — crucial to the supply chains of Huawei and other Chinese tech companies — closer to its backyard, with plans to support a new Taiwan Semiconductor Manufacturing plant in Arizona.Mr. Wang, the foreign minister, urged the United States to step back and seek areas where the two countries can work together. Pessimism about the relationship is nonetheless widespread, though most Chinese officials and analysts blame the Trump administration for trying to deflect attention from its failure to control the pandemic.“It is not difficult to see that under the impact of the coronavirus in this U.S. election year various powers in the U.S. are focused on China,” Zhao Kejin, a professor of international relations at Tsinghua University, wrote in a recent paper. “The China-U.S. relationship faces the most serious moment since the establishment of diplomatic relations.”While he eschewed the idea of a new Cold War, his alternative phrasing was no more reassuring: “The new reality is China-U.S. relations are not entering ‘a new Cold War’ but sliding into a ‘soft war.’”Reporting and research were contributed by Claire Fu in Beijing, Lin Qiqing in Shanghai and Motoko Rich in Tokyo.