EVER since the Nationalist KMT, the losing side in the Chinese civil war, fled to Taiwan in 1949, China's Communist rulers have reserved the right to take back by force what they see as a renegade province. When America broke off diplomatic relations with Taiwan in 1979 and recognised China instead, Congress passed a law obliging the administration to “provide Taiwan with arms of a defensive character” to guard against a hostile mainland.

That support seems to be wobbling. This week Barack Obama agreed to refurbish Taiwan's ageing fleet of F-16 fighter jets (see article), but Chinese objections made the deal less advantageous than it would have been. Meanwhile, a small but influential chorus of academics and policymakers is arguing that these should be America's last arms sales to Taiwan.

What has changed to justify this shift? Little in Taiwan itself. These days the country is a thriving democracy, worthier of support than the dictatorship it was when American backing was rock solid. Nor does Taiwan look better able to defend itself. The main shift in the military balance across the Taiwan Strait in recent years has been a massive one in China's favour. More than 1,000 missiles on its eastern seaboard now point at Taiwan, and China's navy and air force have hugely expanded. Refitting the old F-16s is a token gesture, and China knows it.

Turning a paler shade of green

Two main arguments are made in America to justify abandoning Taiwan. The first is that its ally is now a strategic liability. Under the “blue” (KMT) president, Ma Ying-jeou, cross-straits relations are better than they have ever been. But the “green” opposition is more nationalistic. The fear is that one day Taiwan will make a formal declaration of independence. China says it will respond to that with force. Some in America fret that in backing Taiwan, the United States risks being dragged into conflict, even nuclear war.

How realistic is that fear? Under the previous green president, Chen Shui-bian, Taiwan's relations with both China and America plumbed new lows. Mr Chen's successor as leader of the greens, Tsai Ing-wen, is running against Mr Ma in the presidential election in January. But she is a lot more moderate than Mr Chen, and the provocateurs who want to declare formal independence are mainly old and fading. Younger green politicians may be nationalistic, but they seem more pragmatic and understand the imperative of American support.

The second argument is that, even if it never came to war, Taiwan would still be an obstacle to better Sino-American relations. Give China what it wants, runs this line of thinking, and it will co-operate more on a host of issues ranging from nuclear proliferation to climate change. Rather than provoking China by arming Taiwan and patrolling the seas, it would be better to placate it, and throw it the morsel of Taiwan.

But to walk away from Taiwan would in effect mean ceding to China the terms of unification. Over the long run, that will not improve Sino-American relations. Five thousand years of Chinese diplomatic history suggest it is more likely to respect a strong state than a weak and vacillating one. Appeasement would also probably increase China's appetite for regional domination. Its “core interests” in the area seem to be growing. To Chinese military planners, Taiwan is a potential base from which to push out into the Pacific. At minimum, that would unsettle Japan to the north and the Philippines to the south.

Strong American backing for Taiwan has served the region well so far. It has improved, rather than damaged, cross-straits relations, for Mr Ma would never have felt able to open up to China without it, and it has been the foundation for half a century of peace and security throughout East Asia (see Banyan). To abandon Taiwan now would bring out the worst in China, and lead the region's democracies to worry that America might be willing to let them swing too. That is why, as long as China insists on the right to use force in Taiwan, America should continue to support the island.