Iranian Supreme Leader Ayatollah Ali Khamenei speaks in Tehran. (Associated Press)

Brent Scowcroft was national security adviser to Presidents Gerald Ford and George H.W. Bush.

Congress again faces a momentous decision regarding U.S. policy toward the Middle East. The forthcoming vote on the nuclear deal between the P5+1 and Iran (known as the Joint Comprehensive Plan of Action, or JCPOA) will show the world whether the United States has the will and sense of responsibility to help stabilize the Middle East, or whether it will contribute to further turmoil, including the possible spread of nuclear weapons. Strong words perhaps, but clear language is helpful in the cacophony of today’s media.

In my view, the JCPOA meets the key objective, shared by recent administrations of both parties, that Iran limit itself to a strictly civilian nuclear program with unprecedented verification and monitoring by the International Atomic Energy Agency and the U.N. Security Council. Iran has committed to never developing or acquiring a nuclear weapon; the deal ensures that this will be the case for at least 15 years and likely longer, unless Iran repudiates the inspection regime and its commitments under the Treaty on the Non-Proliferation of Nuclear Weapons and Additional Protocol.

There is no more credible expert on nuclear weapons than Energy Secretary Ernest Moniz, who led the technical negotiating team. When he asserts that the JCPOA blocks each of Iran’s pathways to the fissile material necessary to make a nuclear weapon, responsible people listen. Twenty-nine eminent U.S. nuclear scientists have endorsed Moniz’s assertions.

If the United States could have handed Iran a “take it or leave it” agreement, the terms doubtless would have been more onerous on Iran. But negotiated agreements, the only ones that get signed in times of peace, are compromises by definition. It is what President Reagan did with the Soviet Union on arms control; it is what President Nixon did with China.

Iran has finally reached a nuclear deal with the U.S. and international partners. Here's what's in the deal, and what happens next. (Gillian Brockell and Julio C. Negron/The Washington Post)

And as was the case with specific agreements with the Soviet Union and China, we will continue to have significant differences with Iran on important issues, including human rights, support for terrorist groups and meddling in the internal affairs of neighbors. We must never tire of working to persuade Iran to change its behavior on these issues, and countering it where necessary. And while I believe the JCPOA, if implemented scrupulously by Iran, will help engage Tehran constructively on regional issues, we must always remember that its sole purpose is to halt the country’s nuclear weapons activities.

Israel’s security, an abiding U.S. concern, will be enhanced by the full implementation of the nuclear deal. Iran is fully implementing the interim agreement that has placed strict limits on its nuclear program since January 2014 while the final agreement was being negotiated. If Iran demonstrates the same resolve under the JCPOA, the world will be a much safer place. And if it does not, we will know in time to react appropriately.

Let us not forget that Israel is the only country in the Middle East with overwhelming retaliatory capability. I have no doubt that Iran’s leaders are well aware of Israel’s military capabilities. Similarly, the Gulf Cooperation Council (GCC) members have impressive conventional militaries, and the United States is committed to enhancing their capabilities.

Congress rightfully is conducting a full review and hearing from proponents and opponents of the nuclear deal. However, the seeming effort to make the JCPOA the ultimate test of Congress’s commitment to Israel is probably unprecedented in the annals of relations between two vibrant democracies. Let us be clear: There is no credible alternative were Congress to prevent U.S. participation in the nuclear deal. If we walk away, we walk away alone. The world’s leading powers worked together effectively because of U.S. leadership. To turn our back on this accomplishment would be an abdication of the United States’ unique role and responsibility, incurring justified dismay among our allies and friends. We would lose all leverage over Iran’s nuclear activities. The international sanctions regime would dissolve. And no member of Congress should be under the illusion that another U.S. invasion of the Middle East would be helpful.

So I urge strongly that Congress support this agreement. But there is more that Congress should do. Implementation and verification will be the key to success, and Congress has an important role. It should ensure that the International Atomic Energy Agency, other relevant bodies and U.S. intelligence agencies have all the resources necessary to facilitate inspection and monitor compliance. Congress should ensure that military assistance, ballistic missile defense and training commitments that the United States made to GCC leaders at Camp David in May are fully funded and implemented without delay. And it should ensure that the United States works closely with the GCC and other allies to moderate Iranian behavior in the region, countering it where necessary.

My generation is on the sidelines of policymaking now; this is a natural development. But decades of experience strongly suggest that there are epochal moments that should not be squandered. President Nixon realized it with China. Presidents Reagan and George H.W. Bush realized it with the Soviet Union. And I believe we face it with Iran today.