Threat of a global currency war
It may not really be a currency war, but even I was surprised by the aggressive language being used by senior American and Chinese officials in Washington last week. Not to mention the head of the IMF. It's been a long time since economic relations between the major powers have been this bad-tempered.
I reported from the "frontline" last Thursday, but here's an "idiot's guide" to the debate over global currencies, which I've just done for the World Service.
Coming out of the financial crisis, every country wants to grow as fast it can. That's not the problem. The problem is how.
The United States and Britain have the largest budget deficits in the G20 - which means they're looking at years of cuts. They're looking for exports to pick up the slack, and the best way to boost exports is through a weaker currency.
The problem is that the eurozone wants the same thing. So does Japan. And so does China - even though America and the eurozone think it's time that the Chinese consumer stepped up to the plate.
It sounds like a global price war, with each country fighting to under-bid the other. But when companies have price wars - don't we consumers usually win?
The trouble is that exchange rates aren't the same as prices - if the dollar is going down, then other currencies have to go up. And governments aren't companies: if they don't like where their currency is going they can intervene. The rest of the world is left fighting the price war on its own.
That is exactly what China and other Asian exporting countries have been doing for the last few years - they've spent hundreds of billions of dollars fixing the market to keep their exports cheap. More than a trillion, in the case of China, which now sits on a mountain of dollar reserves.
At the start of the summer, China promised to let its exchange rate go up - but since then it has strengthened against the dollar by just 2%. The yuan has fallen about 10% against the euro and the yen. You can see why the US and other governments gathered in Washington last week were less than thrilled.
China says the focus on the exchange rate misses the point - policy-makers should focus on the why the US saves too little as a nation, and Asian economies save too much. Long term, that IS what re-balancing the global economy must be about.
But, as the director of the International Monetary Fund said last week, you can't go many steps along that road without a substantial change in exchange rates.
That's why you should worry about where talk of currency wars will lead. Because if the world's leaders cannot agree on the role that currencies will play in this global economy - they're not going to agree on very much else.