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Arms control agreements are not usually struck between friendly nations. They are either signed by defeated or frightened states, or by enemies who view them as beneficial to their national security despite ongoing enmity.

South Africa gave up its secret nuclear weapons program only after its apartheid government was ousted.

Libya’s Muammar Qaddafi renounced his nuclear bomb effort only after America’s 2003 invasion of Iraq made him fear that Libya might be next.

The United States and the former Soviet Union negotiated nuclear arms treaties that substantially reduced the nuclear peril to both while still committed to each other’s destruction.

Support for sanctions is weakening. U.S. allies who initially endorsed such economic pressure may resist continuing to apply what initially brought Iran to the table. Walking away from this deal would leave the U.S., not Iran isolated.

President Obama acknowledged at a rare press conference Tuesday that the July 14th agreement would not transform Iran into a “liberal democracy.”

It will not end Teheran’s use and support of terrorism to advance its national interests.

It will not stop its sponsorship of Hezbollah, the Shiite proxy which runs much of Lebanon, or its aid to Hamas, the militant Sunni Palestinian group which controls Gaza.

It is unlikely to prompt Iran to abandon the Houthis who are tearing Yemen apart, or persuade Iran to relinquish its dominance of Baghdad or fight ISIS more vigorously in Syria. And no, it will not free the three American hostages whom it has imprisoned on bogus charges, or require the mullahs to disclose the fate of a fourth American who disappeared there.

Iran is likely to continue its quest for regional hegemony and its ruthless oppression of its people.

Finally, the accord will not require Iran to dismantle its nuclear infrastructure that could eventually produce a weapon if the Supreme Leader decides to do so, its most serious flaw. Obama did not admit this, but his initial goal was always unrealistic.

But if vigorously implemented, the agreement could substantially reduce the size and scope of Iran’s nuclear efforts and retard its march to a nuclear bomb by at least 10-15 years in exchange for the lifting of sanctions and the restoration of over $140 billion in frozen assets and funds to the Iranian regime.  As Secretary of State John Kerry has emphasized:  “This is a nuclear deal.”

Such a deal, Team Obama has argued, would make America and the world safer. And that would be something.

The last four U.S. presidents have tried and failed to persuade Iran to scale back its nuclear efforts.  If successful -- a big if --  the agreement could expand Iran’s “breakout”  -- the amount of time it needs to renege on its nonproliferation pledges and race to a bomb – from two-to-three months to about a year.

While the 159-page agreement is complex, technically tough to read, and may still contain unpleasant surprises, it reduces the number of spinning enrichment centrifuges by two-thirds and eliminates 98 percent of Iran’s low-enriched uranium. It requires Iran to transform a plant that makes plutonium, another nuclear weapons fuel, into a benign research and production facility. It would provide 24/7 surveillance of Iran’s declared nuclear facilities and a process by which inspectors could demand access to suspect sites.

Aaron David Miller, a frequent critic of Obama’s foreign policy, called the deal the “least-worst of a series of bad options,” a “narrowly focused business deal designed to defuse a short-term problem — Iran’s nuclear weapons ambitions.”

Yes, hopes that the Iranian regime may change in 5 or 8 or 10 or 15 years when some of its provisions expire are thrown in for good measure, but they are not really the point of this accord. Nor should critics demand that better behavior by Iran or closer ties with the West be its outcome. Arms control agreements are intended to buy time until a nation comes to its WMD senses – that is, either changes its behavior or changes its regime.

The accord has worrisome mysteries and troublesome provisions. Precisely how the international inspectors and Iran have decided to resolve outstanding questions about its nuclear program’s “previous military dimensions” has not been disclosed. And critics have blasted the phased lifting of the embargo on the sale of ballistic missiles and other conventional arms, a concession Teheran demanded late in the haggling in Vienna. Just last week Gen. Martin Dempsey, the outgoing chairman of the Joint Chiefs of Staff, opposed such a move.

But White House officials have been touting provisions at the heart of this and other arms control pacts: namely, the IAEA’s ability to verify that Iran has finally stopped cheating and whether sanctions can be re-imposed or “snapped back” on the regime should the inspectors conclude that Teheran is still up to its tired old tricks.

While the agreement does not give IAEA inspectors immediate “anywhere, anytime” access to undeclared or military sites, it does create a process that is likely to assure such access in under 24 days. A special eight-member commission -- the European Union, U.S., UK, France, Germany, Russia, China, and Iran — would review IAEA requests for access.

It would take only a “consensus,” or 5 of the 8 members, to demand access. So Iran, Russia and China could not vote to keep inspectors out.

If Iran is determined to be cheating, sanctions on Iran would automatically “snapback” unless the United Nations Security Council specifically approves a new resolution (which Washington could veto) to keep the suspension in place. Even Max Boot, a harsh critic of Iran and of much of Obama’s foreign policy, calls this “snapback” provision the agreement’s “most pleasant surprise.”

Kenneth Pollack, a former CIA analyst and Persian Gulf expert at the Brookings Institute is also impressed by the verification and “snapback “ mechanisms. While several provisions of the agreement were weaker than he would have liked and expected, he said, both key parts of the agreement were “what we wanted, not what the Iranians wanted.” On balance, he said, the deal imposes some “meaningful constraints that will make it hard for Iran to cheat, as well as some meaningful disincentives for Iran not to cheat.”

And then there's this: Alternatives to this agreement are likely to be worse, Obama has repeatedly argued. Support for sanctions is weakening. U.S. allies who initially endorsed such economic pressure may resist continuing to apply what initially brought Iran to the table. Walking away from this deal would leave the U.S., not Iran isolated.

Finally, force has not been ruled out as a tested method of stopping Iran should it cheat or break out. While Obama has clearly rejected a military option, Israeli Prime Minister Benjamin Netanyahu made clear on Tuesday that for Israel, it remains not only alive, but, he implied, preferable to what he assailed as Obama’s historically bad agreement.

Hopefully, Teheran was listening. For if history is any guide, only an agreement whose violation is backed by the threat of force is likely to command Iran’s full attention.