President Trump has done business with royals from Saudi Arabia for at least 20 years, since he sold the Plaza Hotel to a partnership formed by a Saudi prince. Mr. Trump has earned millions of dollars from the United Arab Emirates for putting his name on a golf course, with a second soon to open.

He has never entered the booming market in neighboring Qatar, however, despite years of trying.

Now a feud has broken out among these three crucial American allies, and Mr. Trump has thrown his weight firmly behind the two countries where he has business ties, raising new concerns about the appearance of a conflict between his public role and his financial incentives…

“Other countries in the Middle East see what is happening and may think, ‘We should be opening golf courses’ or ‘We should be buying rooms at the Trump International,’” said Brian Egan, a State Department legal adviser under the Obama administration. “Even if there is no nefarious intent on behalf of the president or the Trumps, for a president to be making money from business holdings in sensitive places around the world is likely to have an impact.”