Donald Trump is hardly the first American president to slap unilateral tariffs on imports.
Every inhabitant of the Oval Office since Jimmy Carter has imposed some kind of protectionist curbs on trade, often on steel.
Nor will Mr Trump’s vow to put 25% tariffs on steel and 10% on aluminium by themselves wreck the economy: they account for 2% of last year’s $2.4trn of goods imports, or 0.2% of GDP.
If this were the extent of Mr Trump’s protectionism, it would simply be an act of senseless self-harm.
In fact, it is a potential disaster-both for America and for the world economy.
As yet it is unclear exactly what Mr Trump will do.
But the omens are bad.
Unlike his predecessors, Mr Trump is a long-standing sceptic of free trade.
He has sneered at the multilateral trading system, which he sees as a bad deal for America.
His administration is chaotic, and Gary Cohn’s ominous decision on March 6th to resign as the president’s chief economic adviser deprives the White House of a rare free-trader, signalling that it has fallen into protectionist hands.
Not since its inception at the end of the Second World War has the global trading system faced such danger.
This danger has several dimensions.
One is the risk of tit-for-tat escalation.
After the EU said it would retaliate with sanctions on American goods, including bourbon and Harley-Davidson motorbikes, Mr Trump threatened exports of European cars.
The second danger springs from Mr Trump’s rationale.
The tariffs are based on a little-used law that lets a president protect industry on grounds of national security.
That excuse is self-evidently spurious.
Most of America’s imports of steel come from Canada, the European Union, Mexico and South Korea, America’s allies.
Canada and Mexico look set to be temporarily excluded-but only because Mr Trump wants leverage in his renegotiation of the North American Free-Trade Agreement, which has nothing to do with national security.
Mr Trump is setting a precedent that other countries are sure to exploit to protect their own producers, just as spuriously.
It is not clear whether other countries can respond legally when national security is invoked in this way.
This puts the World Trade Organisation (WTO) into a rat trap.
Either Mr Trump will provoke a free-for-all of recrimination and retaliation that the WTO’s courts cannot adjudicate, or the courts will second-guess America’s national-security needs, in which case Mr Trump may storm out of the organisation altogether.
Mr Trump has lobbed his grenade as the WTO is already under strain.
The collapse of the Doha round of trade talks in 2015, after14 fruitless years, put needed reforms on hold indefinitely.
Disputes that might have been swept into a new trade round have fallen to the WTO’s dispute-resolution machinery, which is too slow and too frail to carry the burden.
The WTO has not kept pace with economic change.
Investment is increasingly tied up in intangibles, such as patents and copy right, rather than physical assets, such as steel mills.
