We both were elected to the Senate in 1978 and privileged to have served together on the Senate Armed Services Committee for 30 years, during which we each held committee leadership positions of chairman or ranking minority member. We support the Iran Agreement negotiated by the United States and other leading world powers for many reasons, including its limitations on Iran’s nuclear activities, its strong inspections regime, and the ability to quickly re-impose sanctions should Iran violate its provisions.

But we also see a compelling reason to support the agreement that has gotten little attention: Rejecting it would weaken the deterrent value of America’s military option.

As former chairmen of the Senate Armed Services Committee, we have always believed that the U.S. should keep a strong military option on the table. If Iran pursues a nuclear weapon, some believe that military action is inevitable if we’re to prevent it from reaching its goal. We don’t subscribe to that notion, but we are skeptical that, should Iran attempt to consider moving to a nuclear weapon, we could deter them from pursuing it through economic sanctions alone.

How does rejecting the agreement give America a weaker military hand to play? Let’s imagine a world in which the United States rejects the nuclear accord that all other parties have embraced. The sanctions now in place would likely not be maintained and enforced by all the parties to the agreement, so those would lose their strong deterrent value. Iran would effectively argue to the world that it had been willing to negotiate an agreement, only to have that agreement rejected by a recalcitrant America.

In that world, should we find credible evidence that Iran is starting to move toward a nuclear weapon, the United States would almost certainly consider use of the military option to stop that program. But it’s highly unlikely that our traditional European allies, let alone China and Russia, would support the use of the military option since we had undermined the diplomatic path. Iran surely would know this, and so from the start, would have less fear of a military option than if it faced a unified coalition.

While the United States would certainly provide the greatest combat power in any military action, allies and other partners make valuable contributions—not just in direct participation, but also in access rights, logistics, intelligence, and other critical support. If we reject the agreement, we risk isolating ourselves and damaging our ability to assemble the strongest possible coalition to stop Iran.

In short, then, rejecting the Iran deal would erode the current deterrent value of the military option, making it more likely Iran might choose to pursue a nuclear weapon,and would then make it more costly for the U.S. to mount any subsequent military operation. It would tie the hands of any future president trying to build international participation and support for military force against Iran should that be necessary.

Those who think the use of force against Iran is almost inevitable should want the military option to be as credible and effective as possible, both as a deterrent to Iran’s nuclear ambitions and in destroying Iran’s nuclear weapons program should that become necessary. For that to be the case, the United States needs to be a party to the agreement rather than being the cause of its collapse.

In our many years on the Armed Services Committee, we saw time and again how America is stronger when we fight alongside allies. Iran must constantly be kept aware that a collective framework of deterrence stands resolute, and that if credible evidence evolves that Iran is taking steps towards a nuclear arsenal, it would face the real possibility of military action by a unified coalition of nations to stop their efforts.

The deal on the table is a strong agreement on many counts, and it leaves in place the robust deterrence and credibility of a military option. We urge our former colleagues not to take any action which would undermine the deterrent value of a coalition that participates in and could support the use of a military option. The failure of the United States to join the agreement would have that effect.

Former Senator Carl Levin, Democrat of Michigan, and former Senator John Warner, Republican of Virginia, both served as chairmen of the Senate Armed Services Committee.

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