I'm starting to think maybe the Iran nuclear deal was a mistake

This is not what happened on either side of the Middle East’s sectarian divide. Instead, the deal has more often contributed to escalating tensions. In retrospect, this was foreseeable: Iran was perfectly capable of projecting power across the region with or without a nuclear arsenal. As for its rivals, they never trusted Iran’s assurances and saw warming relations between Tehran and Washington as a new and potentially even greater threat.

Following the nuclear deal, Saudi Arabia and the Gulf monarchies, with aid from the U.S., stepped up their involvement in the tragic and destructive war in Yemen to counter perceived Iranian encroachment in their backyard. The Saudis also exacerbated the region’s sectarian tensions by executing a prominent Shiite cleric in January 2016. This, predictably, led to the ransacking of the Saudi embassy in Tehran and the cutting off of diplomatic relations between the two countries. The Saudi moves were viewed as a reaction to warming U.S. ties with Iran. As political scientist and Mideast analyst Marc Lynch wrote, “Saudi Arabia views Iran’s reintegration into the international order and its evolving relationship with Washington as a profound threat to its own regional position. Mobilizing anti-Shiite sectarianism is a familiar move in its effort to sustain Iranian containment and isolation.”

At the same time, rather than moderating its regional ambitions as the JCPOA’s proponents might have hoped, Iran has spent the years since the deal was signed supporting a network of Shiite militias in Iraq, Syria, Lebanon, Yemen, and other countries, part of a larger project to, as BuzzFeed’s Borzou Daragahi put it, “establish territorial dominance from the Gulf of Aden to the shores of the Mediterranean.” Iran might have done all this regardless. But it was also responding to the Saudi actions. Either way, there’s certainly no evidence that nuclear diplomacy, or the lack of a nuclear weapon, has helped the neighbors overcome their differences.