On Thursday, Donald Trump announced a 25% tariff on steel and a 10% tariff on aluminum, likely excepting Canada and Mexico – and perhaps America’s strategic partner Australia in due course. It was, of course, a shocking thing.

When was the last time a US president did such a shocking thing? Well, actually it was in the first week of March 2002, at exactly the same point in a presidential first term. The president was George W Bush, of the Republican party, and the steel tariff he imposed was 30%. And before that? Ronald Reagan, with his “voluntary export restraints”.

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As the man from Vaudeville ran out on stage to announce: don’t clap too loud, it’s an old theater.

A difference is that both Reagan and Bush were ideological free-traders, globalists, union-busters, dedicated to the approved ideology in these matters. So when they imposed tariffs, they largely got a pass – except from a few principled free traders on their own staffs, who did complain, as Gary Cohn has done now.

As a declared protectionist, on the other hand, when Trump makes a protectionist move, zero tolerance applies. But … since when is a 25% tariff higher than a 30% tariff? (Hint: it’s not).

The practical effect of the tariffs will be small. The steel industry has about 150,000 employees in total; the aluminum industry claims about the same amount. No producer will commit the billions required to expand capacity, knowing that tariffs come and go; the Bush tariffs were taken off after only two years. If there is spare capacity that can be brought into service under the tariff barrier, that will show mainly as increased productivity rather than many new jobs.

But there won’t be much increased production either, because there won’t be that much effect on imports or even on import costs. If Mexico and Canada (and eventually Australia) are exempted, does anything stop them from increasing their own exports to the US, while also increasing imports from the non-exempt countries – such as Brazil – to meet their domestic needs? Maybe, but then it’s not clear how they can be considered “exempt”. So the tariffs – as they are currently planned – could amount to little more than a game of metallurgical musical chairs.

And even where they bite – as they will in some cases – will they slow things down in the face of the fiscal expansion already under way? Not by much.

So why did Trump do it? The reasons are obvious.

First reason: politics. It’s March of Trump’s midterm election year. He needs to be able to say he’s delivered on his promises to American workers. Now he can say that.

Second reason: politics. There is a special election in western Pennsylvania on Tuesday, and it’s widely believed that the Republican candidate is weak, in a district Trump carried by 20 points. The tariff announcement can’t hurt with that one.

Third reason: also politics. Right on cue, Trump’s enemies chimed in. The all-knowing economists. The Wall Street globalists. The financial press. It was a great instance of what the Brits call Project Fear. The tariffs will bring on a trade war! They will cripple the construction industry! They are “stupid”! Even the ever-popular president of the European Commission Mr Juncker stepped up, threatening retaliation against Harley-Davidson and Levi-Strauss – although not against Citigroup or Goldman Sachs. Take that, America!

Of course in the White House, all this just makes the president laugh. He’s looking at his poll numbers; confident they’ll go up; the midterms are coming into focus and things are right on track.