But, in truth, this was no caprice. Israel and Washington have been heading in this direction for a while.
In Israel, there is cross-party support for keeping the Golan.
Michael Oren, a former Israeli ambassador to the US and a confidant of Mr Netanyahu's, formally launched a plan last year to quadruple the size of the Golan's settler population, to 100,000, within a decade.
The US State Department offered its apparent seal of approval last month when it included the Golan Heights for the first time in the "Israel" section of its annual human rights report.
This month, senior Republican senator Lindsey Graham made a very public tour of the Golan in an Israeli military helicopter, alongside Mr Netanyahu and David Friedman, Mr Trump's ambassador to Israel. Mr Graham said he and fellow senator Ted Cruz would lobby the US president to change the territory's status.
Mr Trump, meanwhile, has made no secret of his disdain for international law. This month, his officials barred entry to the US to staff from the International Criminal Court, based in The Hague, who are investigating US war crimes in Afghanistan.
The ICC has made enemies of both Washington and Israel in its initial, and meagre, attempts to hold the two to account.
Whatever Mr Netanyahu's spin about the need to avert an Iranian threat, Israel has other, more concrete reasons for holding on to the Golan.
The territory is rich in water sources and provides Israel with decisive control over the Sea of Galilee, a large freshwater lake that is crucially important in a region facing ever greater water shortages.
The 1,200 square kilometres of stolen land is being aggressively exploited, from burgeoning vineyards and apple orchards to a tourism industry that, in winter, includes the snow-covered slopes of Mount Hermon.
As noted by Who Profits, an Israeli human rights organisation, in a report this month, Israeli and US companies are also setting up commercial wind farms to sell electricity.
And Israel has been quietly co-operating with US energy giant Genie to explore potentially large oil reserves under the Golan. Mr Trump's adviser and son-in-law, Jared Kushner, has family investments in Genie. But extracting the oil will be difficult, unless Israel can plausibly argue that it has sovereignty over the territory.
For decades the US had regularly arm-twisted Israel to enter a mix of public and back-channel peace talks with Syria. Just three years ago, Barack Obama supported a UN Security Council rebuke to Mr Netanyahu for stating that Israel would never relinquish the Golan.
Now Mr Trump has given a green light for Israel to hold on to it permanently.
But, whatever he says, the decision will not bring security for Israel, or regional stability. In fact, it makes a nonsense of Mr Trump's "deal of the century" a regional peace plan to end the Israeli-Palestinian conflict that, according to rumour, may be unveiled soon after the Israeli election.
Instead, US recognition will prove a boon for the Israeli right, which has been clamouring to annex vast areas of the West Bank and thereby drive a final nail into the coffin of the two-state solution.
Israel's right can now plausibly argue: "If Mr Trump has consented to our illegal seizure of the Golan, why not also our theft of the West Bank?"
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