AS PRESIDENT-ELECT, Donald Trump has developed an unnerving practice of casually overturning decades-old tenets of U.S. foreign policy. First he had a phone call with Taiwan’s president, the first such contact by a president or president-elect since 1979. Now he has just as breezily signaled that he will blow up long-standing U.S. Middle East policy. Last week, Mr. Trump nominated his former bankruptcy attorney, David M. Friedman, as his ambassador to Israel, and Mr. Friedman quickly issued a statement affirming that he would work from a U.S. Embassy relocated to “Israel’s eternal capital, Jerusalem.”

Like the chat with the Taiwanese president, the democratically elected leader of a U.S. ally, Mr. Trump’s apparent plan to move the embassy has a certain common sense. West Jerusalem has been the seat of the Israeli government since 1949, and though the United States officially regards the city’s status as unresolved, there’s little doubt that Israel would retain the west in any peace settlement. Moving the embassy there would not preclude a Palestinian capital on the city’s predominately Arab eastern side.

Yet the two previous presidents who promised during their election campaigns to move the embassy — George W. Bush and Bill Clinton — later backed off for good reason. They were worried a mostly symbolic step would touch off a backlash against Israel and the United States across the Arab world, damage the quiet improvement in relations between the Jewish state and Sunni Arab regimes, and possibly touch off violence by Palestinians. It would also damage U.S. standing to broker peace talks between Israelis and Palestinians — a role that Mr. Trump recently suggested he was interested in.

It’s hard to know if Mr. Trump has considered these potential consequences, but with Mr. Friedman as his adviser, we’d guess not. The New York attorney, who represented Mr. Trump when his Atlantic City casinos collapsed in debt, is a funder and advocate of West Bank Jewish settlement who has disparaged the two-state solution promoted by the past three American presidents. As Israeli commentators have pointed out, he stands distinctly to the right of Israeli Prime Minister Benjamin Netanyahu, who reaffirmed his own support for “two states for two peoples” just days before Mr. Friedman’s appointment.

Mr. Friedman also has a penchant for launching vicious personal attacks on those who disagree with him — and then doubling down. When he was criticized for comparing supporters of the liberal American Jewish group J Street to the collaborating Jewish “kapos” of Nazi death camps, he responded by saying they were actually “far worse than kapos.” It’s hard to imagine how Mr. Friedman, who has no diplomatic experience, will forge constructive relations with the majority of Israelis who don’t share his hard-line ideology.

Mr. Trump, for his part, might be seen as delivering a refreshing shake-up to some ossified U.S. positions. But shake-ups can lead to unexpected and dangerous fallout, especially in high-tension regions such as the Middle East. The disturbing question about the president-elect is whether he has given any thought to such consequences.