President Donald Trump's move to recognise Jerusalem as Israel's capital on Wednesday could have deep repercussions across the region.
Any recognition of Israel's control over the city will be welcomed by Israel, a close American ally, and be popular with pro-Israel evangelical Christian voters who make up a key part of Trump's base.
But it could also trigger violence in the region, derail a developing US Mideast peace plan before it even gets off the ground and infuriate key allies in the Arab world and in the West.
Here is a look at why Jerusalem is such a sensitive issue:
Israel claims all of Jerusalem as its capital, while the Palestinians claim the city's eastern sector, captured by Israel in the 1967 Mideast war, as the capital of a future independent state.
These rival claims lie at the heart of the decades-long Israeli-Palestinian conflict.
The conflict is focused largely on the Old City, home to Jerusalem's most important Jewish, Christian and Muslim holy sites, and in particular on a hilltop compound revered by Jews and Muslims.
The compound, known to Jews as the Temple Mount, is the spot where the biblical Jewish Temples stood thousands of years ago and is considered the holiest site in Judaism.
Today, it is home to the Al Aqsa Mosque, the third-holiest site in Islam, and the iconic gold-topped Dome of the Rock.
While Israel controls the city and its government is based there, its annexation of east Jerusalem is not internationally recognised.
The international community overwhelmingly says the final status of Jerusalem should be resolved through negotiations.
Why is Trump doing this?
On the campaign trail, Trump took a strongly pro-Israel stance and promised to relocate the U.S. Embassy from Tel Aviv, where most countries keep their embassies, to Jerusalem. Since taking office, he has learned that such a move is easier to talk about than to carry out.
Under American law, the president must sign a waiver every six months that leaves the embassy in Tel Aviv.
In June, Trump renewed the waiver, as a string of predecessors has done. This week, another six-month deadline passed without Trump renewing it.
US officials say Trump will again sign the waiver but will also instruct the State Department on Wednesday to begin the multi-year process of moving the US Embassy to the holy city.
The officials say the recognition of Jerusalem as Israel's capital will be an acknowledgement of “historical and current reality” rather than a political statement but that moving the embassy will not happen immediately.
The officials spoke to reporters on condition of anonymity because they were not authorized to publicly discuss Trump's announcement beforehand.
Recognizing Jerusalem as Israel's capital could allow Trump to say that he kept a campaign promise. It also will thrill Israel, whose prime minister, Benjamin Netanyahu, is one of Trump's biggest supporters on the global stage.
What effect will his declaration have?
On the ground, very little will change. Netanyahu's office and official residence are in Jerusalem, as are the country's parliament, Supreme Court and Foreign Ministry. Visiting world leaders immediately travel to Jerusalem for meetings with Israeli officials.
For its residents, Jerusalem is an open city where Jews and Palestinians can move about freely, though in reality interaction between the sides is minimal and there are large disparities between wealthier Jewish neighborhoods and impoverished Palestinian ones.
But a US declaration carries deep symbolic meaning by essentially imposing a solution for one of the core issues in the conflict.
How will this be received?
Beyond the electoral concerns, there seems to be little upside for Trump in making a change.
Trump likes to call an Israeli-Palestinian peace agreement “the ultimate deal,” and he has invested significant effort in laying the groundwork for a peace initiative in the coming months.
His son-in-law and close adviser, Jared Kushner, is leading that effort and a close aide, Jason Greenblatt, has crisscrossed the region for talks with Israelis, Palestinians and other Arab leaders.
The Palestinians have warned that changing the status of Jerusalem would mean the end of those peace efforts. They also have warned of mass street protests something that could easily erupt into full-scale violence.
International opposition to the move, including from key American allies, also has grown increasingly strident. In recent days, the European Union, Germany and France have all implored Trump not to take action on Jerusalem.
The 57-member Organization of Islamic Cooperation said changing Jerusalem's status would amount to “naked aggression” against the Arab and Muslim world, and the head of the Arab League said it would be a “dangerous measure that would have repercussions” across the entire Middle East.
Perhaps most significantly, Saudi Arabia spoke out strongly against the possible American step. The Saudis are a key American ally necessary for any attempt to forge a region-wide peace.
Will there really be violence? Israeli security officials say they are monitoring the situation and prepared for all scenarios. Israel and the Palestinians also maintain discrete security ties in the West Bank that have helped prevent violence from escalating in recent years.
Still, much of the violence between Israel and the Palestinians in Jerusalem and the West Bank over the past 20 years has been connected to tensions in the holy city.
The city experienced deadly riots in 1996 after Israel opened a new tunnel in the Old City. The second Palestinian uprising erupted in 2000 after then-opposition leader Ariel Sharon visited the Temple Mount.
More recently, the city experienced a wave of Palestinian stabbings in late 2015 in part because of growing numbers of visits by Jewish nationalists to the Temple Mount, and last summer, the city again experienced weeks of unrest when Israel tried to install security cameras next to the Al Aqsa Mosque after a Palestinian gunman killed two Israeli police officers.