In the roughly three weeks since Japanese Prime Minister Sanae Takaichi commented on a possible Taiwan contingency in parliament, China has unleashed economic reprisals, nationalist barbs and a diplomatic offensive to show its displeasure.

Now, President Xi Jinping’s government is escalating the dispute with an appeal to the United Nations, a move aimed at pressuring all countries to side with China’s stance on any future conflict over Taiwan — or stay out of its way.

In a letter to UN Secretary-General Antonio Guterres on Friday, Fu Cong — China’s envoy to the global body — accused Takaichi of violating international law with her comments, which publicly linked a Taiwan Strait crisis with the possible deployment of Japanese troops.

“If Japan dared to intervene militarily in the Taiwan Strait, it would be an act of aggression,” Fu wrote. “China will resolutely exercise its right of self-defense under the UN Charter and international law and firmly defend its sovereignty and territorial integrity.”

The letter accused Japan of undermining the “post-war international order,” citing documents that back China’s assertion that it has sovereignty over Taiwan. Xi reinforced that message in a phone call on Monday with U.S. President Donald Trump — their first contact since reaching a broad trade truce on Oct. 30 — with the Chinese leader saying “Taiwan’s return to China is an integral part” of the order after World War II.

“China and the U.S. fought shoulder to shoulder against fascism and militarism,” Xi said, according to a Chinese statement. “Given what is going on, it is even more important for us to jointly safeguard the victory of WWII.”

China’s moves seek to further its claims to Taiwan and widen the dispute beyond Japan in an international body where Beijing enjoys wide support, particularly from developing countries in the Global South. In invoking the right to self-defense and equating a Japanese intervention as an act of aggression, Beijing is effectively asserting no country — including the U.S. — should come to Taiwan’s defense in the event of an invasion.

The letter “could be the first step of China’s new efforts to establish the legal ground and narrative” for a potential military move in the future, including firing at Japanese assets in a conflict, said William Yang, senior analyst for Northeast Asia at the International Crisis Group.

While former President Joe Biden repeatedly stated that the U.S. would defend Taiwan in any attack from China, Beijing carefully calibrated its responses because it knew Washington could “inflict real pain” on Beijing, Yang added. Japan’s economic reliance on China, by contrast, makes it an easier target.

China “wants to see how much would the Trump administration be willing to offer Japan concrete support,” Yang said. He added that Beijing is also “trying to warn other democracies about the consequences of making similar statements on Taiwan.”

Trump’s statement on the call with Xi made no mention of either Taiwan or Japan. He called the U.S.’s relationship with China “extremely strong” and said he accepted an invitation by Xi to visit Beijing in April. Last week, a State Department spokesperson said the U.S. commitment to Japan’s defense remained “unwavering.”

The UN letter is the latest step in China’s effort to rally international support in its escalating spat with Japan. While it isn’t a formal resolution requiring member states to vote, it forces every country to consider where it stands on the issue — and that may be good enough for China.

“It only needs silence, because silence for China means acquiescence, acceptance,” said Alicia Garcia Herrero, chief Asia Pacific economist at Natixis SA, who has researched China’s influence at the UN. “And nobody’s saying this is outrageous. So this is why for China this is already a big win.”

In their broadsides against Takaichi, Chinese officials and state media commentaries have frequently evoked Japan’s wartime aggression against China and other Asian nations, accusing her government of returning to a dangerous path of “militarism.” Fu, the Chinese ambassador to the UN, has also seized on Japan’s alleged transgression in arguing against its bid for a seat on the UN Security Council.

On Friday, the Chinese Embassy in Japan posted on X that China would have the right to carry out “direct military action” without needing authorization from the UN Security Council if Japan took any step toward renewed aggression. That post cited UN Charter clauses regarding “enemy states” during the Second World War, without further elaboration.

Japan refused to let that one slide, pointing out that the UN “enemy state” clauses are now considered obsolete. “We’re hoping China will act and speak responsibly as a major power and permanent security council member of the UN,” Maki Kobayashi, a senior Japanese government spokeswoman, said in a statement.

China’s narrative relies heavily on a legal distinction Beijing has aggressively promoted since Russia’s invasion of Ukraine. Chinese officials, including Foreign Minister Wang Yi, have repeatedly rejected parallels between the two, arguing that while Ukraine is a sovereign state, Taiwan is a territory of China and the dispute is internal.

‘Keep one’s head down’

From this perspective, Beijing has accused the U.S. and its allies of “double standards,” upholding Ukraine’s borders while violating China’s sovereignty over the island. Now by baking this logic into formal UN communications, China is attempting to establish that any foreign military assistance to Taiwan isn’t a defense of democracy, but an illegal violation of Chinese territory.

Russia — China’s “no-limit” partner — has been one of the few countries joining Beijing to reprimand Takaichi, with most others staying on the sidelines. U.S. allies have largely kept quiet so far.

For China, the relentless focus on Takaichi also serves a domestic political purpose: Making Xi look strong by not ceding ground to a historical enemy on one of its most sensitive domestic political issues.

The spat allows China to boost domestic nationalism while also serving to deter any other countries from speaking out on Taiwan, according to Wen-Ti Sung, non-resident fellow at the Atlantic Council‘s Global China Hub.

“By getting other states to vocalize support for China’s position at the UN, China is trying to project legitimacy of its position via strength by numbers,” he said. “When two major economic giants are fighting, the path of least resistance is simply to keep one’s head down.”

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