FEW diplomatic sophisms are as skilfully worded as America’s “one-China policy”. Mere repetition by American officials that their country sticks to it has helped more than anything else to keep the peace between two nuclear-armed powers. Were America to reject the policy, mainland China would be enraged. Anti-American riots would erupt. The government in Beijing might even respond by launching a military attack on Taiwan, or American forces in the region. The global economy would shudder. Millions of lives would be threatened.

Small wonder, then, that pulses quickened on both sides of the Pacific when Donald Trump, as president-elect, questioned the policy. (“I don’t know why we have to be bound by a one-China policy unless we make a deal with China having to do with other things, including trade,” he said.) Last month he changed his mind and reassured China’s president, Xi Jinping, that he would, in fact, uphold it. Yet the one-China policy is in a fragile state. Far from casting doubt on it, Mr Trump needs to make America’s support for the status quo clearer than ever.

The one-China policy is a fudge. At the time it was devised, the governments in Beijing and Taipei both claimed to be the rightful rulers of all China. (Chiang Kai-shek, Taiwan’s leader until 1975, had fled from the mainland in 1949 after losing a civil war against Mao Zedong.) Until the 1970s, America recognised only “Free China”, ie Taiwan. Under the new policy, it acknowledged that both sides believed there was only one China while tactfully not saying who was the rightful ruler of Taiwan. The aim was to butter up Red China, which Richard Nixon wanted as an ally in the cold war against the Soviet Union. The communists would have preferred America to accept what they call the “one-China principle”—namely that Taiwan is a renegade province of China, and ought to bow to the Communist Party. But they were content that America was prepared to switch diplomatic recognition from Taiwan to the People’s Republic and withdraw American troops from the island (both of which it did in 1979). So the fudge stuck.

The result has been an extraordinary relationship between two great powers. They were divided by ideology but united initially by their common hatred of the Soviets and later by their common pursuit of wealth by trading with each other.

However, Taiwan remains a flashpoint (see article). Communist China has not given up its dream of taking control of the island, by force if necessary. America has kept on selling weapons to Taiwan. The “Taiwan Relations Act” requires it to view an attack as a matter of “grave concern” to America: a hint that it might come to Taiwan’s aid. China has often made clear its outrage at this. Its rapid military build-up in recent years has been aimed, not least, at deterring America from trying to defend Taiwan. If it could keep America out, it could, in all probability, inflict a crushing defeat on the island.

One China, one Taiwan

This tinderbox has now been exposed to the spark of Taiwanese democracy. In the 1990s the island began to cast off authoritarianism. The Taiwanese are pragmatic. Last year they elected an independence-leaning president, but one who prefers not to antagonise the communists. Most believe that the island is already autonomous enough. Few want to enrage China by formally declaring independence. But they have also started to question the idea of “one China”. They see the mainland as a different country, and abhor the idea of being swallowed by the giant dictatorship next door. Taiwan has never been ruled by the communists. Since 1895 it has been under the mainland’s control for less than five years.

Most are happy to let the one-China fudge persist. But will it? Having stirred up nationalist feelings for so long, the Communist Party can never abandon its claim. Some day, to shore up its popularity, it may be tempted to invade Taiwan.

America’s ability and willingness to deter China is not only vital to Taiwan but also a measure of its role in the world more broadly. The arms that America sells to Taiwan would not enable the island to hold out for long against a Chinese onslaught, but they are a token that America has a stake in Taiwan’s fate, and that China should beware. Rather than using Taiwan as a bargaining chip, America should maintain its military support for the island. If repeating a misleading mantra is the price of peace, it is worth it.