Donald Trump is telling leaders from across the Middle East that he intends to declare Jerusalem the capital of Israel, an explosive move that will break from 50 years of US foreign policy, potentially derail his administration’s hopes of restarting the Israeli-Palestinian peace process, and threaten to spark violence across the region.

Trump reportedly also told King Abdullah of Jordan and Palestinian President Mahmoud Abbas by phone that he plans to relocate the US embassy from Tel Aviv to Jerusalem. That move won’t be imminent, however. The White House told reporters late Tuesday that the president plans to sign another six-month waiver delaying the embassy move; Trump is expected to publicly announce both decisions on Wednesday.

The administration’s planned announcement is already sparking fury across the Arab world. A spokeswoman for Abbas’s office issued a statement early Tuesday warning of “dangerous consequences” if Trump moves forward with plans to eventually move the embassy. King Abdullah was equally critical, saying in a statement that the White House shift on Jerusalem “will undermine the efforts of the American administration to resume the peace process.”

Right-wing Israeli leaders, by contrast, didn’t try to disguise their happiness. In a message to Trump, Naftali Bennett, the head of the Jewish Home party, said he wanted to thank “you from the bottom of my heart for your commitment and intention to officially recognize Jerusalem as the capital of Israel.”

The sharply divergent reactions highlight the fact that there is almost no other issue in the Middle East as contentious as the future of Jerusalem.

Both the Palestinians and the Israelis claim Jerusalem as their capital. Though Israel’s Parliament and the prime minister’s home are in Jerusalem, they sit in West Jerusalem, on the side of the city Israel has controlled since 1949. Israel captured East Jerusalem in 1967 and annexed that half of the city.

The international community considers East Jerusalem occupied territory. But that half the city also contains sites holy to all three major monotheistic religions, including the Western Wall, the most sacred site in the world for Jews, and Haram al-Sharif (the Noble Sanctuary), a sacred site for Muslims.

The Palestinians would like to officially divide the city and make East Jerusalem the capital of a future Palestinian state. The Israelis, to put it mildly, disagree — and the right-wing government of Israeli Prime Minister Benjamin Netanyahu has long made clear that it wouldn’t even consider making concessions over Jerusalem.

The decades-long political fight over the future of the city is what makes Trump’s new moves so momentous — and so dangerous.

Trump is touching the third rail of the Israeli-Palestinian conflict

The status of Jerusalem has been a source of both division and contention for decades. During most of the 1990s — including during the creation of the Oslo peace accords between the Israelis and the Palestinians — negotiations over the final status of the city were left for the future to avoid derailing the rest of the talks.

In 2000, negotiations between then-Israeli Prime Minister Ehud Barak and then-Palestinian leader Yasser Arafat reportedly came close to dividing the city between the two peoples. The Israelis would have retained control over the Western Wall, and the Palestinians would have been given control over the Temple Mount, the third-holiest site in Islam.

Final negotiations reportedly broke down over questions of who would control of a maze of underground tunnels that run beneath Jerusalem’s Old City.

There have been no recent negotiations over the city for a simple and grim reason: Israeli-Palestinian peace talks have been largely on hold for years, with no indications that they’ll be resuming anytime soon.

In the meantime, Jerusalem has retained the uniquely strange status of a city without a country. Americans born in the city must put “Jerusalem” rather than “Israel” on their passports. That’s because the nationality of the entire city remains contested, a source of deep fury for many Israelis and American Jews.

Trump promised to move the US embassy to Jerusalem. He hasn’t.

As a presidential candidate, Donald Trump promised to move the US embassy from Tel Aviv to Jerusalem, which he called “the eternal capital of the Jewish people.”

And then he didn’t. His campaign promises notwithstanding, in June he signed a six-month waiver keeping the US embassy in Tel Aviv, blocking the Jerusalem Embassy Act of 1995 which would, otherwise, automatically move the embassy to Jerusalem. (It’s the same waiver signed by all three of his predecessors, a legal loophole that allows the president to claim American national security interests require the embassy to stay put in Tel Aviv).

“No one should consider this step to be in any way a retreat from the President’s strong support for Israel,” the White House said in a statement at the time. “President Trump made this decision to maximize the chances of successfully negotiating a deal between the Israelis and Palestinians.”

Despite the significance of his change on the official US position on Jerusalem, Trump seems to again be stopping short of moving the embassy out of Tel Aviv. On Tuesday, Bloomberg News and other media outlets reported that Trump would sign another waiver, further delaying any effort to start the hard task of relocating the US embassy to Jerusalem.

Trump still wants to make the ultimate deal

On Sunday, Trump’s son-in-law and senior adviser Jared Kushner opined on the administration’s commitment to the peace process. “If we’re going to try to create more stability in the region as a whole, you have to solve this issue,” Kushner told the audience at Brookings Institution’s Saban Forum.

The president, he said, “sees this as something that has to be solved.”

But he also hedged on Jerusalem, saying the president had yet to make a decision on the status of the city and the embassy. It’s clear that his comments no longer hold: Trump is now expected to publicly label Jerusalem as the capital of Israel on Wednesday.