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The holiest site in Jerusalem's old city is arguably the most contested piece of land in the world.
It is where Jews go to pray at the Western Wall, and directly above Muslims pray at the Al Aqsa mosque. Jews call it the Temple Mount because it is where King Solomon is said to have built the first Jewish temple, demolished around 586 BC.
But Muslims call it Haram al-Sharif — the Noble Sanctuary — from where they believe the Prophet Mohammed once ascended to heaven.
So it is understandable that both sides in the Israel Palestinian conflict want the ancient city of Jerusalem as their capital.
It also explains why US recognition of Jerusalem as Israel's capital city would be such an incendiary move, especially coming from a President who has made so much of his promise to advance the Israel-Palestinian peace process.
But that pledge will be meaningless if, as some predict, Donald Trump announces unilateral recognition later today.
It would immediately undermine the peace process by placing Israeli claims above the Palestinians'. And it would effectively end the Palestinians' dream of Jerusalem becoming even a shared capital under a two-state solution.
For Palestinians, it remains unthinkable that their capital could be anywhere else.
Embassy move would break with decades of US policy
The expected announcement would also fly in the face of international law, that for decades has ruled it is illegal for Israel to declare Jerusalem its capital.
In its 1947 partition plan for Palestine, the United Nations voted to place Jerusalem under a "permanent international regime" administered by the UN. Neither side could claim the city as its capital.
Although that did not stop Israel's first prime minister David Ben Gurion doing just that, when in 1949 he rejected the UN resolution and declared Jerusalem to be Israel's "eternal capital".
The UN's stance remains unchanged today, even after Israel captured the old city and East Jerusalem from Jordan during the 1967 Six-Day War, and in 1980 passed a law that went even further — declaring Jerusalem as Israel's eternal and "undivided" capital, meaning no part of the city could ever be "transferred to a foreign government or body".
In the decades since, the international community — including the US — has refused to recognise Israel's sovereignty over Jerusalem.
The US, Australia and other foreign nations have instead kept their embassies at Tel Aviv, and maintained only consulates in Jerusalem.
Although Mr Trump — since last year's election campaign — has also flagged the possibility of moving the US embassy to Jerusalem.
Moving the US embassy alone would be provocative enough, even if it involved nothing more than changing the existing sign from "US Consulate" to "US Embassy". It would break with decades of US policy.
Every US president since 1995 has signed a national security waiver every six months to keep the US embassy in Tel Aviv.
Even if Donald Trump signs the waiver this month, some in his own administration say it is a matter of when, and not if, the embassy will move to Jerusalem.
Trump: The president who destroys chances of peace?
But formally recognising Jerusalem as Israel's capital represents far more and could ultimately change the course of Middle East peace.
The status of Jerusalem has long been one of the most intractable sticking points to a peace agreement. And given its sensitivity, successive US presidents have unanimously insisted that the city's status must be decided in negotiations.
The US — in recognising Jerusalem as Israel's capital, and Israel's alone — would have sided against the Palestinians. It would automatically relinquish its longstanding role as independent mediator.
To say such a scenario could spark a new round of violence is an understatement. Already leaders across the Arab world have warned of a backlash.
The Palestinian President Mahmoud Abbas's Fatah party has called for mass protests to "confront this aggression".
The Palestinian Liberation Organisation says it would represent the "kiss of death" to the two-state solution.
Turkey says it would result in "catastrophe and new conflict". Jordan has flagged an emergency meeting of Arab leaders within days.
France's President Emmanuel Macron has phoned the US President warning him not to change the status of Jerusalem. And Jordan's Foreign Minister has given a similar warning to the US Secretary of State Rex Tillerson.
Indeed, it is difficult to know what Donald Trump could hope to achieve by announcing such formal recognition. Even inside his administration there is opposition to the idea.
Several US officials have privately expressed concern at the implications, not just for Middle East peace but for US security interests across the region.
US embassies in Jerusalem and beyond would undoubtedly become a target for extremists.
And Donald Trump would surely go down in history as the US president who damaged — if not destroyed — the chances of Middle East peace.
Topics: foreign-affairs, world-politics, unrest-conflict-and-war, religion-and-beliefs, judaism, islam, donald-trump, israel, palestinian-territory-occupied
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