There are two widespread and competing interpretations of Donald Trump’s behaviour. Many of his fans view him as a genius – artfully “draining the swamp” while playing his opponents like a fiddle. Many of his critics cleave to the opposite view – that he’s mad, or an idiot, or a mad idiot, making decisions on the basis of the latest thing he saw on TV, whatever personal gripe occupies his thoughts, and whatever whim captures his tweeting thumb at any given moment.

The evidence seems to contradict each of these positions to an extent. If you believe he’s the Stradivarius of strategy, even to the point of thinking his most grossly offensive moments are deliberate and beneficial in some way, then it’s still hard to explain how he has already lost so many of his advisors to various scandals, and left himself open to increasingly serious allegations about his campaign’s links with the Kremlin.

Similarly, while it’s evidently the case that a troubling proportion of the things he does are ill-judged and petulant, if you think he is solely a copper-bottomed loon, thrashing around on the floor of the Oval Office with a smartphone in one hand and the nuclear button in the other, then it makes it all the more difficult to justify the fact that he won the selection, then the election, and has managed somehow to keep together a sizeable chunk of his original support base. His approval rating still stands at 45 per cent.

If anything, it seems that while he behaves like an erratic man-child much of the time, at other times he does act rationally – at least on the measure of his own interests. That might stick in the craw of some to admit, but it’s true – and there’s danger in blindly assuming his every action is illogical and barmy.

Trump’s decision to recognise Jerusalem as the capital of Israel, and move the US embassy there accordingly, is a good example. It is easy for outside observers who disapprove of the decision to write it off as just some more insanity from the White House. But there are three ways in which this makes plenty of sense for him politically.

First, he’s keeping a campaign promise – one which others have failed to honour. It’s still central to Trump’s message and campaigning identity that he is different to all the others in American politics. The riff of Mr Smith Goes to Washington remains a powerful archetype, and he is still acting out his very own remake (except this is a version where Mr Smith likes gold lifts and sometimes insults the disabled). Plenty of Republicans, in particular, have talked about moving the embassy and recognising Jerusalem, but they’ve all backed off under the weight of official advice and international disapproval. For this President, going ahead in spite of those things is a positive benefit, reinforcing his claim to authenticity.

Second, plenty in Congress agree with him. It’s over 20 years since both houses passed the Jerusalem Embassy Act, which authorises exactly this step. Clinton, and his successors, refused to implement it, but the Act has never been repealed. The fact remains that there are a lot of other politicians in Washington who agree with Trump about this specific issue, including some who abhor his politics more generally.

Third, the prospect of violence in the Arab world strengthens, not weakens, Trumpism. If Clinton, Bush and Obama all held back from this step for fear that it would spark condemnation, rioting, terrorism and possibly hostile state action in the Middle East, none of those things are a particular deterrent to this President. In “America First” terms, on what basis would he accept that the US ought to limit itself for fear that others – least of all Muslims – might disagree with its decisions about its own embassy? If the Arab world responds to this decision with mob violence, or terrorist atrocities, hen he will surely simply argue that this proves he is right to build a wall, and to implement a travel ban on various Muslim majority countries.

So disapprove all you like of Trump’s decision on Jerusalem, but don’t mistake it for a Twitter spasm or a temper tantrum. It serves his purposes – and so does everyone’s loud disapproval of it.