THIS is how wars usually start: with a steadily escalating stand-off over something intrinsically worthless. So don't be too surprised if the US and Japan go to war with China next year over the uninhabited rocks that Japan calls the Senkakus and China calls the Diaoyu islands. And don't assume the war would be contained and short.

Of course we should all hope that common sense prevails.

It seems almost laughably unthinkable that the world's three richest countries - two of them nuclear-armed - would go to war over something so trivial. But that is to confuse what starts a war with what causes it. The Greek historian Thucydides first explained the difference almost 2500 years ago. He wrote that the catastrophic Peloponnesian War started from a spat between Athens and one of Sparta's allies over a relatively insignificant dispute. But what caused the war was something much graver: the growing wealth and power of Athens, and the fear this caused in Sparta.

The analogy with Asia today is uncomfortably close and not at all reassuring. No one in 431BC really wanted a war, but when Athens threatened one of Sparta's allies over a disputed colony, the Spartans felt they had to intervene. They feared that to step back in the face of Athens' growing power would fatally compromise Sparta's position in the Greek world, and concede supremacy to Athens.

The Senkakus issue is likewise a symptom of tensions whose cause lies elsewhere, in China's growing challenge to America's long-standing leadership in Asia, and America's response. In the past few years China has become both markedly stronger and notably more assertive. America has countered with the strategic pivot to Asia. Now, China is pushing back against President Barack Obama's pivot by targeting Japan in the Senkakus.