It is hard to find a Middle Eastern official betraying signs of anxiety over what President-elect Donald Trump will do once in office. From Egypt to Turkey to Saudi Arabia to, unsurprisingly, Israel, government leaders delight in discussing what an improvement the Trump presidency is likely to be over President Barack Obama’s record in the region.

A key reason for this honeymoon is the extraordinary vagueness of the views expressed by Mr. Trump about the Middle East’s many intricate and intertwined conflicts, some of them festering for decades. This intellectual vacuum, in turn, has allowed many regional leaders to imagine that a Trump administration will take their side in the struggles that are tearing the Middle East apart.

“Everybody is projecting,” said Fouad Siniora, the former prime minister of Lebanon and the leader of the Sunni bloc in its parliament. “That’s because they don’t have real clarity of what is the position of the next administration of the United States. They are hopeful and they are expecting.”

The danger of this situation, of course, is that the Middle East’s politics is largely a zero-sum game—and that some of these countries will be inevitably disappointed, and will react accordingly, sometimes in ways that hurt their neighbors and the U.S.

The few positions that Mr. Trump and his aides articulated on the Middle East are often mutually contradictory. In Syria, he has spoken about aligning with Russia and even the regime of President Bashar al-Assad against Islamic State—statements that were greeted with joy by regime officials in Damascus. At the same time, he pledged a more confrontational policy against Mr. Assad’s main supporter Iran—a position that has given hope to the Syrian rebels and to their backers in Saudi Arabia.