THERESA May appears to have been spared one of Donald Trump’s power handshakes when they got together at the G20 summit in Hamburg at the weekend. But then it was soft news they had to announce afterwards, the prospect of a trade deal between their two countries that would come “very, very quickly”. A pity Trump then diverted attention from these glad tidings with an irrelevant aside about his new friendship with Vladimir Putin. It showed where in his own mind the priorities lay.

May is now back in London and, in an attempt to relaunch her crumbling premiership, appealed for all-party support in Brexit. At the same time there are rumours of fresh Tory plots to unseat her – these two political ploys may not be unconnected. Summits tend to bring out the vainglorious side of national leaders: the world’s press hang on their every word, while carping domestic critics have been left far behind. I seem to remember there was another woman Prime Minister who went to a summit, that one in Paris, and came out to make a significant announcement. Days later (27 years ago) she was gone — from Downing Street and from history. Just a thought.

Of course this woman prime minister’s achievements are rather flimsier than that woman prime minister’s, and Hamburg offers a good example. Trade deals are not concluded by the national leader’s utterance of sweet nothings at press conferences. In the UK, it is true, foreign treaties are a matter of the royal prerogative. In other words the government of the day simply decides what it is going to agree with a partner country, and that is that. There is no obligation even to debate the terms of the treaty at Westminster, let alone vote on them.

In the US, on the other hand, it is the Congress that makes, and on occasion breaks, foreign treaties. Heaven help the partner country that does not take account of, say, the beggarly chicken farmers of Arkansas or the struggling steel industry of Ohio, because the filibusters in their defence will wax long and eloquent, perhaps with the complete works of Shakespeare thrown in for good measure.

Trump can no more command the Congress to approve a trade deal with the UK than he could command the Department of Homeland Security to turn away Muslims seeking to enter his country. Well, he did try as much, but the federal courts ruled he was acting unconstitutionally. The American system puts far greater restraints on executive abuse than the parliament at Westminster does. A president accustomed to talking before thinking brings this out nicely.

In any trade deal with the UK, the Congress will want its pound of flesh. The US takes 17 per cent of British exports (against 44 per cent going to the EU) but sends only eight per cent of our imports. With Brexit likely to damage our European trade, there should be scope for Americans to sell more to us. If I were one of the president’s advisers or a Republican congressman, that is what I would be aiming at. Quid pro quo is perhaps the best the Brits can hope for.

Meanwhile we should reflect, too, that this is a process which will need to be repeated dozens of times after Brexit, as we replace by a new accord each individual treaty through which the EU regulates its exchanges in the rest of the world – and with an acute shortage of experienced trade negotiators too. Is this a process that can be completed before the end of the century?

The global context in which it must all take place formed a doleful backdrop to the summit in Hamburg, lent some local colour by the anti-globalist protesters’ burning of cars and looting of shops. Chancellor Angela Merkel, who — poor woman — had staged this event as one of the launchpads for her campaign to be re-elected in October, doggedly claimed that the 20 presidents and prime ministers present in Hamburg rejected protectionism.

But the very idea was belied by the rapprochement between Trump and Putin that overshadowed everything else. Most of these summits are soon forgotten, yet this one may be remembered just for that.

Putin is certainly not interested in free trade: he runs an exploitative kleptocracy. And Trump is explicitly a protectionist president, of a type familiar in US history, whatever merely verbal concessions Merkel may have wrung out of him. Look rather at what he does than at what he says. He got to the White House by standing on a platform of America First. Not yet in the letter but in the spirit, he has torn up the North American Free Trade Agreement, especially in its openness to the Mexicans. He has repudiated (before congressional approval could be obtained) the Trans-Pacific Partnership with 11 other nations. He claims that in such agreements the US has been taken advantage of, and that the cost can be counted in American jobs. Now the balance is to be tipped – which presumably must mean there will be fewer jobs for other nationalities. Is this to be a criterion for the US-UK trade agreement too?

The most fundamental fact of all is that we are still suffering the consequences of the great financial crash of 2007-8, which seems destined to live on in popular consciousness just as the slump of the 1930s did. But nowadays at least some nations are trying to avoid the terrible trap into which nearly all nations fell 80 years ago, of trying to cure unemployment through protectionism. This led only to the beggar-my-neighbour policies that made the overall situation worse, and heightened the tensions that led at length to the Second World War. This, whatever other catastrophes it brought on the world, at least solved the industrial problem by requiring the production needed for victory.

As the Hamburg summit showed, the international community is now divided between those, like Merkel, who remember such lessons and those, like Trump, who have forgotten them or never knew them. It seems to me that Theresa May, and, for that matter, most other British politicians, are just thoroughly mixed up. On the one hand the UK still claims for itself its historic role as a free-trading nation (though not in respect of the free movement of labour which is an essential part of that role). On the other hand, it is renouncing its place in the biggest free-trading zone available to it. Once on the outside, in a world increasingly organised in huge trading blocs, I fear it will be hard for us to find a place as beneficial as the one we are so recklessly abandoning.

In the previous protectionist era, it was the small nations that in the end fared worst. They could not escape the economic consequences, and in the political crises and wars that followed many of them lost their liberty and independence. Similarly, today the situation for Scotland seems to me a bit grim.

When it was something like this two centuries ago, Adam Smith showed us the way out. A clear idea of our actual economic situation might help now, but for the most part we lack even that.