Perhaps appropriately, the first heavy rain after weeks of sunshine was s plashing off the paving stones outside bookseller Imad Muna’s news and stationery shop in Saladin, the busy main shopping street of Arab East Jerusalem.

Inside, the proprietor, his 27-year-old son Ahmad, and a 70-year-old customer Adnan Abdel Razeq were holding a well-informed debate on the probable meaning of President Donald Trump’s speech they would be watching on television several hours later. Mr Abdel Razeq pointed out that the UN Security Council had in 1980 roundly declared “null and void” the law passed by the then Israeli Prime Minister Menachem Begin declaring Jerusalem the “complete and united” capital of Israel.

While agreeing with that, Ahmad disputed another of the old man’s points – namely that it would not be so bad if Mr Trump stressed he was specifying West Jerusalem as Israel’s capital which would have left open the possibility that East Jerusalem might still in future be that of the Palestinians. It was a distinction the US President would not make.

Suggesting that Mr Trump might be attempting to provoke Hamas into breaking off reconciliation talks with its Fatah rivals, Ahmad said: “Trump should not be talking about this at all. It is very dangerous. I’m sure there will be big demonstrations.” His father added that for the Palestinians an equal stake in Jerusalem was the “last bottom line.” If Mr Trump was to challenge that, “that’s it for the peace process”.

Given the rhetoric raging among Arab leaders in advance of Mr Trump’s speech, the trio in Occupied East Jerusalem – and it is worth remembering that is how every almost every Western government officially sees it – sounded sober, even restrained. But this seemed bred from a weary but steely determination not to let one more betrayal bully them into acceptance of that 50-year-old occupation.  Even were Riyadh and other Arab leaders tacitly to support Mr Trump, which they did not seem to be doing in the wake of the announcement, “they will never part the Palestinian people from their national aspiration” exclaimed Mr Abdel Razeq.

From a rain sodden Jerusalem, it was hard to figure out just what had possessed the “pyromaniac” – to use the Arab Knesset member Ayman Odeh’s word – Mr Trump to unravel at a stroke the delicate Western consensus on the most inflammatory of all the issues between Israel and the Palestinians. There was no doubt a complex of motives in the decision finally to move the US embassy to Jerusalem in defiance of international edicts dating back to 1948.

But was one the influence of Sheldon Adelson, Mr Trump’s most lavish funder in the 2016 campaign, a zealous supporter of right-wing Israeli nationalism, and the original bankroller of Israel Hayom, the free newspaper which has been Israeli Prime Minister Benjamin Netanyahu’s most unflinching cheerleader?

What is clear is that if this was an attempt for Mr Trump to reconcile his dream of the “ultimate deal” with the search for a campaign promise to fulfil, at whatever cost, the latter won.

Some of this must have gathered momentum in the apparently cordial dialogue – maintained according to some diplomats by WhatsApp – between two fabulously rich young men, Mr Trump’s son-in-law Jared Kushner and Saudi Arabia’s Crown Prince Mohammed bin Salman. It appears that this may have given birth to the idea – first reported in The New York Times – of an emerging Trump-planned “peace” deal. This would, among other things, have offered the Palestinians a capital away from the heart of East Jerusalem proper, in the suburb of Abu Dis. “This was first mentioned after [the] Oslo accords [in the early nineties], Mr Abdel Razeq said of the reports, “and we laughed about it then.”

If Crown Prince Mohammed bin Salman really thought – and it has not of course been confirmed – that he could ignore the Palestinians’ “last bottom line”, then he may have seriously underestimated the extent to the place Jerusalem holds in the hearts of not only Palestinians – but those throughout the Muslim world, including his own country. That may well be why the King of Saudi Arabia has appeared to reject the formula allegedly thrashed out with Mr Kushner, instead reinforcing the long held Arab Peace Initiative under which Israel would be recognised throughout the Arab world in return for a Palestinian state in Gaza, the West Bank and East Jerusalem, and based on June 1967 borders.

It’s hardly a secret that Saudi Arabia and Israel are seeking to nurture an alliance to challenge their common opponent Iran. Perhaps some in the Kingdom were prepared to forsake the Palestinians in pursuit of that cause. But Jerusalem is at the heart of why they dare not do that.

The heroically optimistic glosses on Trump’s announcement on Wednesday afternoon pointed to his stated recommitment to a negotiated peace if the two sides agreed its terms – rather ignoring the fact that almost every utterance by members of Israeli Prime Minister Mr Netanyahu’s coalition government – including the Prime Minister himself – has suggested that Israel is not prepared for any deal a Palestinian leader could literally – let alone politically – live with. It was true too that he did not repeat the phrase beloved of the Israeli right – Jerusalem as the “eternal and undivided capital” of Israel, at least in theory opening the possibility of East Jerusalem as a Palestinian capital.

But nor – and this is a striking point – did he simply specify, as all governments in the past including his own have assumed in the past it would be – that West Jerusalem would be the site of a future of an Israel capital. It was hard in other words to escape the conclusion that his administration was indeed also open to Jerusalem as an “undivided capital.”

The glaring fallacy in Trump’s logic is that because not recognising Jerusalem as the capital or moving the US embassy there had not produced peace – as if there were no other obstacles – doing both things might. Does he think that the Palestinians will somehow agree to utterly humiliating terms in order to prevent the construction of the embassy being completed? If so, he simply doesn’t understand the conflict. The immediate results of this lack of understanding remain to be seen. As Mr Abdel Razeq said, much will depend on how Israeli security forces, now being busily deployed in advance of Friday prayers, handle the promised Palestinian “days of rage”.

One of the wisest Israelis in Jerusalem is Daniel Seidemann, the man who has advised successive US and other foreign governments on the hugely delicate issues the city presents for any peace process. He warned ahead of Mr Trump’s announcement that it could spell the end of the US’s role as a peace broker. “Jerusalem is a kind and dignified place if it’s treated with respect,” he told me. “But those who ignore its complexities find it comes back to bite their sensitive parts.”

That may be what happed to the Saudi Crown Prince. But it doesn’t seem to have bothered Mr Trump, even as the State Department warns its diplomats of the security risks to their missions in the Arab world from protests against his announcement.