The problematic word, of course, is “perceived”. On trade, as on other subjects, Trump’s perceptions generally don’t seem very tethered to reality. And to be fair, the economic policy performance of other major players, notably the EU, hasn’t been noteworthy for the triumph of clear economic thinking.

Still, there have been a number of attempts to model trade war over the years, relying on one of two approaches. The first is to imagine that governments actually do maximize national income, or perhaps an objective function that gives extra weight to well-organized interest groups. The second appeals to the historical experience of the world before international trade agreements became the norm. Fortunately, these approaches suggest similar tariff levels.

On the first approach: any country large enough that it can affect world prices of the goods it exports, the goods it imports, or both, has an “optimal tariff” greater than zero. The reason is that by limiting its trade, such a country can improve its terms of trade: the price of its exports relative to the price of its imports. This raises real income, other things equal. And the optimal tariff trades off the costs of reduced trade – e.g., the cost of producing goods domestically when they could be purchased more cheaply abroad – against the gains from improved terms of trade.

The problem is that if everyone does this, you get the costs of reduced trade without the benefit of improved terms of trade, because other countries are doing unto you the same thing you’re trying to do unto them. So you end up in a situation of “optimal tariff warfare”, which is actually more like an arms race than a shooting war, in the sense that there’s (usually) no victor and no resolution, just a lot of wasted resources.

So how do you estimate the effects of optimal tariff warfare? You need a “computable general equilibrium” model of world trade – something that shows how production and trade flows depend on tariff rates, calibrated to match the actual data. Then you have to find an equilibrium (a Nash equilibrium, for readers of “A Beautiful Mind”) in which each country is charging its optimal tariff given what everyone else is doing.

There are many assumptions and imputations involved, with the results depending a lot on how easily goods from one country can be substituted for goods from another – a parameter that’s hard to estimate. Still, there are several fairly recent efforts: Ossa finds that a trade war would, under his favored assumptions, lead to tariffs of nearly 60 percent, while Nicita et al, using slightly different assumptions, estimate a rise in tariffs of 32 percentage points from current levels.