Tehran — As President Trump was being feted in the palaces of the Saudi royal family after concluding a historic arms deal, Iranians were celebrating the outcome of a hard-fought election. The vote manifested the determination of Iran’s electorate to continue on the path of moderation and constructive engagement based on mutual respect that brought the world the nuclear deal in 2015.

If past performance is an indicator of future success, another $110 billion worth of weapons will neither reduce “the burden” on the American military nor support “the long-term security of Saudi Arabia,” as the State Department argues. The last time the Saudis spent that kind of money was when they provided billions to the Iraqi dictator Saddam Hussein in the 1980s to arm his war of aggression against Iran. Look what that bought them and the world.

At best, Mr. Trump is extorting our Saudi neighbors, milking them for money they do not have. At worst, he could be turning the United States into Saudi Arabia’s mercenary in the Middle East, a rather ignominious position for America considering where 15 of the Sept. 11 hijackers came from. A security crackdown in Saudi Arabia before Mr. Trump’s visit — as well as the Bahraini regime’s deadly attack on a sit-in immediately afterward — suggest that the region’s despots feel that they’ve been given carte blanche to stamp out peaceful dissent.

In other words, something deeply rotten is unfolding in our part of the world.

To avoid the spread of terrorism and militant extremism, responsible leaders in regional and world capitals alike must step up and tackle the momentous challenges at hand. Sword dances and lavish banquets aside, fundamental contradictions must be addressed.