The US will move its embassy in Israel from Tel Aviv to Jerusalem today.
Donald Trump announced back in December that he would formally recognise Jerusalem as the capital of Israel, but critics say it could create more instability in the region.
Bruce Riedel, a former CIA officer, told CNN he thinks the move is “very dangerous”.
Expressing concern about tensions between Israel and Iran, he said: “[Trump] is only a few days away from throwing another can of gasoline on the fire by moving the embassy to Jerusalem.”
But Ron Dermer, the Israeli Ambassador to the US, said: “I think it’s the right thing to do.”
Israeli Prime Minister Benjamin Netanyahu said the move “advances peace”, but many disagree, with EU ambassadors expected to boycott today’s celebrations.
The US first attempted to move the embassy in 1995, but no president has agreed to do it until Trump.
What makes the move so contentious is that Palestinians and Israelis both believe Jerusalem to be their capital, and it has religious significance for Jews, Christians and Muslims, and Palestinians see east Jerusalem as their future capital.
The US State Department released a statement saying the move reflects “reality”.
"As the President stated on December 6, 2017, the historic opening of our embassy recognizes the reality that Jerusalem is the capital of Israel and the seat of its government," it said.
Ivanka Trump and her husband Jared Kushner, who are both advisers to Trump, will attend.
The Palestine Liberation Organisation condemned the decision, saying the move "gives life to a religious conflict instead of a dignified peace.
"Tragically, the US administration has chosen to side with Israel's exclusivist claims over a city that has for centuries been sacred to all faiths,” they said.