The best plan I've seen is one that Gunner pointed me to a couple of weeks ago, proposed by Warren Buffet in an editorial somewhere. It involves issuing trade credits to countries that buy our products, which authorize them to sell an equal amount back to us. I had been thinking in terms of an offset system, which every country in the world that buys civilian aircraft or military hardware from the US uses on us, but Buffet's plan has an added benefit: The credits themselves could be traded on world markets, which would have the effect of erasing some of the anti-competitive price advantages that result from extremely low wages (China's, for example) that are held down artificially by government manipulation.
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Theory, theory. The US currently is running a $460 billion trade deficit. Part of it is the result of other countries buying our Treasury bonds to pump up the value of the dollar and to keep their currencies low, to give them a trade advantage. Japan practically patented this method 30 years ago and all of the Asian Tigers, plus China, have picked up on it. You can't have free trade in such an environment. But enacting currency controls would be a cure worse than the disease.
The theory of Comparative Advantage, which is the underlying idea to our "free-trade" policies, is itself based on an assumption of perfectly balanced trade. In fact, its arithmetic was worked out originally on an assumption that trade would be all barter, or its money equivalent -- zero balances of trade.
Dumping and subsidizing are, in theory, destructive only to the country that engages in them. But the practice is quite different. They tend to be market-grabbing techniques and they've been very effective.
However, as you say, they're hard to prove and they'll always be there. The only way to short-circuit them that I've seen is something like Buffet's plan.
We don't need tariffs. We need balanced trade. And we don't have to manipulate anyone else's market. We only have to take some control of ours.