Were Iran to withdraw too
Firstly, most observers agree that Iran would resume its nuclear weapons programme, which could quickly lead to the proliferation of nuclear weapons in the Middle East. This would also increase regional tension and a growing sense of insecurity by other countries in the area, which is a recipe for sparking new and further intensifying current violent conflicts.
Secondly, given the intense enmity between Israel and Iran, the Israeli government might well decide to pre-emptively attack Iranʹs nuclear facilities before Tehran reaches breakout point. This would more than likely lead to an Iranian-Israeli war and pull other countries, including the U.S., into the fray, which could have dreadful consequences throughout the Middle East.
Thirdly, such development would also deepen Iranʹs resolve to further entrench itself in Syria, which is precisely what Israel wants to avoid. This too will prompt Israel, as it has done in the past, to attack Iranian military installations in Syria, which could also escalate into a regional conflagration.
Fourthly, Iran will have every reason to accelerate its ballistic missile programme, which poses an even greater danger, not only to Israel but to the U.S.ʹ allies throughout the region. In addition, Iran will have further incentive to increase its financial support of extremist groups looking to destabilise the region, which it has and will continue to exploit.
Fifthly, the unilateral withdrawal from the deal by the U.S. will undoubtedly create a schism with the U.S.ʹ allies – not to mention Russia and China – and could thus foreclose any opening to modify the deal, which Trump failed to consider.
Finally, the United Statesʹ credibility will be seriously tarnished with both friends and nemeses, especially at this juncture when the U.S. is preparing to work out a deal on de-nuclearisation with North Koreaʹs Chairman Kim Jong Un, who now has a perfectly legitimate reason to doubt any American commitment to adhere to future agreements.
Had the deal been maintained
Iranʹs threat perception originates from its sense of encirclement, compelling it to pursue a defensive policy. I believe that Iran would have been willing to re-negotiate various provisions of the deal to prevent it from acquiring nuclear weapons now or at any time in the future, given the assurance that the new deal would first and foremost preserve the regime and that the U.S. would commit to not seeking regime change, now or at any time in the future.