The United Arab Emirates has fewer citizens than Rhode Island, but its de facto ruler, Crown Prince Mohammed bin Zayed of Abu Dhabi, is widely regarded as one of the most influential Arab leaders in Washington and across the Middle East.

He may be the richest man in the world: He controls sovereign wealth funds worth $1.3 trillion, more than any other country. His military is the most potent of any Arab state. His influence in the United States is legendary — and never more felt than under President Trump.

Prince Mohammed, 58, is obsessed with two enemies, Iran and the Muslim Brotherhood, and Mr. Trump has sought to move strongly against both. In fact, the president has repeatedly adopted positions favored by the prince over the reservations of cabinet members and career national security officials on subjects including Iran, Qatar, Libya, Saudi Arabia and the Muslim Brotherhood.

[Read our full report on Prince Mohammed.]

Now Prince Mohammed has caught the interest of American prosecutors as well. The special counsel investigating Russian interference in the 2016 election unearthed evidence that the prince tried to help the Russians open back channels to Mr. Trump’s inner circle.