However isolated North Korea may be, it has long had one major ally: China. But for two years, China’s leader, President Xi Jinping, seemed to be favoring Pyongyang’s neighbor and nemesis to the south.
He spent much political capital wooing South Korea’s president, Park Geun-hye, in hopes of drawing the country away from its longtime ally, the United States. He made an elaborate state visit to Seoul while shunning North Korea and its young leader, Kim Jong-un, whom he has yet to meet. Ms. Park returned the favor last year, coming to Beijing for a major military parade at Tiananmen Square, the only leader of an American ally to attend.
But on Friday, it became clear that Mr. Xi’s efforts had fallen short. In announcing plans to deploy an advanced American missile defense system on South Korean soil, Ms. Park’s government showed that it was embracing its alliance with Washington more than ever, and that it would rely less on China to keep North Korea and its growing nuclear arsenal at bay.
In Beijing, the decision was seen as a major setback, one that went beyond its interests on the Korean Peninsula to the larger strategic question of an arms race in Northeast Asia that could impel China — and Russia — to develop more sophisticated weapons.
Analysts said the deployment of the so-called Terminal High-Altitude Area Defense system, or Thaad, would reinforce the already high level of mistrust in United States-China relations as the Obama administration nears its end, adding to the raw nerves over disputes in the South China Sea and differences over American business access to the Chinese market.
And North Korea, an issue on which there had been some common ground between the two powers — at least when it came to the latest round of United Nations sanctions — is likely to become a greater source of irritation, as China loses an incentive to be tougher on the regime.
In announcing the new missile system, which has been under discussion for several years, the top commander of the American military in South Korea, Gen. Vincent K. Brooks, said on Friday that it was needed to protect South Korea from the North’s nuclear weapons.
But Chinese officials have repeatedly said that they do not believe the North Korean threat is the true reason for the American-initiated deployment. Rather, they say, the purpose of the Thaad system, which detects and intercepts incoming missiles at high altitudes, is to track missiles launched from China.
Now that the system’s implementation has been confirmed, China will almost certainly consider developing more advanced missiles as a countermeasure, said Cheng Xiaohe, an associate professor at Renmin University in Beijing and a North Korea expert.
“A way to deal with Thaad — a shield — is to sharpen your spear,” Mr. Cheng said.
The possibility of the Thaad deployment has bedeviled relations between Washington and Beijing for more than a year. The Chinese renewed their objections at the recent Strategic and Economic Dialogue in Beijing, a meeting of American and Chinese senior officials that was attended by Secretary of State John Kerry.
Last month, Mr. Xi and President Vladimir Putin of Russia made a point of denouncing the Thaad system during Mr. Putin’s visit to Beijing, equating it with the American-built Aegis Ashore ballistic missile defense system deployed in some NATO countries. The implicit message was that the United States was trying to encircle China in the same way that, according to Mr. Putin, it was trying to contain Russia.
Before Mr. Putin’s visit, China’s foreign minister, Wang Yi, expressed the Chinese view that the Thaad system is a strategic game-changer in Northeast Asia.
“The Thaad system has far exceeded the need for defense in the Korean Peninsula and will undermine the security interests of China and Russia, shatter the regional strategic balance and trigger an arms race,” Mr. Wang said. China understands South Korea’s “rational need” for defense, he said, “but we can’t understand and we will not accept why they made a deployment exceeding the need.”
Chinese analysts have said that they expect Japan to eventually deploy Thaad as well, in what they say would be an American attempt to draw it closer into a three-way alliance with South Korea. So far, Japan has shown little interest in the Thaad system, but Washington and Tokyo are jointly working on a new missile interceptor that is expected to start production in 2017.
Talks between Seoul and Washington on the Thaad deployment picked up speed after North Korea conducted its fourth nuclear test in January. After that test, which Pyongyang claimed was of a hydrogen bomb, Ms. Park tried but failed to reach Mr. Xi by telephone, according to South Korean news reports that were later confirmed by Chinese officials.
The nuclear test left Ms. Park convinced that Mr. Xi could not do enough to rein in North Korea’s nuclear ambitions, and that China was uninterested in supporting her “trustpolitik” strategy of finding ways to engage with the North while responding strongly to provocations, South Korean officials said.
In March, South Korea and the United States began formal talks on the Thaad deployment. China tried to persuade Ms. Park to accommodate Beijing’s interests by asking for technical adjustments to the system, under which its radar would penetrate less deeply into China, according to Wu Xinbo, the director of the Center for American Studies at Fudan University in Shanghai. But those adjustments were not made, he said.
Some in South Korea have expressed concern that China, the country’s top trading partner, might engage in economic retaliation for the Thaad deployment. Cheong Seong-chang, a senior analyst with the Sejong Institute in Seongnam, south of Seoul, said China could reduce the number of tourists it allows into the country or even boycott some South Korean goods. Anti-South Korean sentiment in China could also hurt South Korean cultural exports, like the country’s popular films and TV dramas, Mr. Cheong said.
Mr. Wu, the Chinese analyst, said Beijing was unlikely to take such measures in this period of slower economic growth. But he said the debate over North Korea among senior Chinese leaders would almost certainly be reshaped, with officials who favor better relations with Pyongyang gaining more influence, after two years of Mr. Xi keeping its isolated neighbor at a distance.
“The school in favor of a more balanced approach to North Korea will get more sway,” Mr. Wu said.