Their inability to detect clandestine atomic programs in Iraq and North Korea shadows Western officials as they seek to curb Iran's known nuclear activities and keep it from pursuing others in secret.
Closing loopholes in the current nonproliferation system is a key aim for the United States and its five big power partners as they negotiate an agreement with Iran to constrain its nuclear program in return for relief from economic sanctions.
Iran and the major powers missed Tuesday's deadline for a final deal and have given themselves until July 7 to reach one.
Some of the toughest issues that remain are the pace of sanctions relief and the monitoring and verification steps that major powers want to ensure Iran does not cheat on any accord.
In a sign of verification's importance, the International Atomic Energy Agency UN nuclear watchdog said its head, Yukiya Amano, was going to Tehran to discuss the issue, as well as the past potential military dimensions of Iran's program.
U.S. officials say they want to remedy a key weakness in the international nonproliferation regime that allows a country to drag out negotiations with the IAEA indefinitely for access to suspect sites.
Without timely access, including, if necessary to military sites - which Iran's supreme leader said is a red line that he will not cross - finding out if a state is hiding a nuclear program becomes a markedly harder challenge.
"I fail to see how any agreement can pass muster ... that doesn’t have snap inspections (at all sites)," former U.S. deputy secretary of State Richard Armitage told Reuters. "I don’t see how you can have a verifiable regime without having snap inspections - whenever, wherever."
Other analysts said it was unrealistic to expect a country, except possibly one defeated in war, to give inspectors carte blanche to go anywhere they pleased, including military sites.
Iranian officials have expressed concern that UN inspectors could be used to gather intelligence for the United States and its allies, something that happened in Iraq in the late 1990s with the now defunct Iraq special inspection team.
The experience of Iraq after the 1991 Gulf war is an example of the limits the nonproliferation system that then existed.
While combing through the country after a U.S.-led coalition drove Iraqi forces out of Kuwait in 1991, UN arms control inspectors found that Baghdad had been pursuing a secret uranium enrichment program that it had not declared.
That illustrated the inadequacy of comprehensive safeguards agreements under the 1970 Nuclear Non-proliferation Treaty (NPT), which limited the IAEA's verification work to the nuclear sites and materials that a state had declared.
"We discovered almost accidentally in the process of the war with Iraq that they had a secret enrichment program," said Carnegie Endowment for International Peace nonproliferation expert George Perkovich.
North Korea's pursuit of a secret uranium enrichment program even as it capped its known plutonium program is another case in point. Plutonium and highly enriched uranium are fissile materials, the essential ingredient for an atomic bomb.
Chiefly as a result of the Iraqi experience, world powers tried to tighten the nonproliferation system with the creation, in 1997, of the Additional Protocol, a voluntary agreement.
The Additional Protocol widened the scope of verification and increased the IAEA's ability to look for secret nuclear sites by giving it the authority to visit any facility to investigate questions about a state's declarations, according to the nonprofit Arms Control Association group.
But it, too, has its holes. Where there were disagreements between the IAEA and a given state to suspect sites, the two would negotiate how to address the agencies concerns, whether it would be allowed "managed access" and under what conditions.
"Managed access" is a mechanism to allow the minimum needed IAEA oversight to ensure there is no diversion to clandestine nuclear or nuclear-related activities while limiting IAEA access to protect a nation's legitimate military or industrial secrets.
However, there is no time limit on such negotiations, allowing the state to drag the talks out forever if it wished.
"If they cannot come to an agreement, that discussion – that negotiation – can go on ad infinitum," a senior U.S. official said. "We have added a procedure in this agreement that will ensure that that discussion comes to an end," the official added, declining to comment on how quickly that would happen.
"The Iranians need to know we aren't going in there like cowboys and look under the beds. Of course, it's unpleasant for the Iranians to come clean on all their dirty linen and there is a lot of dirty linen," said an official from a country involved in the talks. "The agency just needs the means to do its work."
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