The FX market is about to blow up in the Fed’s face, and there’s nothing they can do about it.

Foreign exchange (FX) is a zero-sum game: if one currency weakens, another must strengthen. Since the value of a currency is relative to other currencies, all currencies can’t weaken together: at least one currency must strengthen as others weaken.

That one strengthening currency has been the U.S. dollar (USD) since mid-2014. The USD has strengthened by 20%, while the Japanese yen and the euro weakened by 20%. Many developing-economy currencies (rand, peso, real, etc.) have fallen off a cliff, suffering 40% to 50% (or even more) declines against the U.S. dollar.

Why does any of this matter? Simply put, the stock market is a monkey on a leash held by central banks–just give the leash a little tug, and the monkey jumps. Bonds are a gorilla–harder to control, but still manageable–but foreign exchange is King Kong, trading $5 trillion a day and impossible to control beyond short-term manipulations.

Currencies set the underlying trend, not just for bonds and stocks, but for entire economies. A weakening currency makes a nation’s exports cheaper in other countries, and the theory is that expanding exports will boost the overall economy–especially if that economy is stagnating or in recession.

A weakening currency also makes imports more expensive in the domestic economy, pushing inflation higher–precisely what every central bank in the world desires, on the theory that inflation will make people spend more (since their money is losing value) and reduce the costs of borrowing (which is presumed to stimulate more borrowing and spending).

This is why everybody seems to want a weaker currency. But as noted above, every currency can’t go down; if some weaken, others have to strengthen.

Which brings us to the current brewing crisis: beneath the propaganda that all is well in the world, the soaring dollar has destabilized the global economy in subtle ways: carry trades have been thrown over, capital flows have reversed, commodities priced in dollars have tanked, and so on.