Iran and Saudi Arabia have been bitter rivals for decades, vying to lead competing branches of Islam and standing on opposing sides of conflicts in Syria and Yemen. But Tehran and Riyadh have taken major steps to de-escalate tensions and boost cooperation, a move that appeared unthinkable until recently. The rapprochement has coincided with growing fears of an all-out war in the Middle East, where U.S. ally Israel is engaged in wars against Iranian-backed groups in the Gaza Strip and Lebanon. The detente process has intensified since Donald Trump’s…