Both national intelligence agencies (NIAs) and international verification organizations (IVOs) attempt to assess compliance with arms control treaties. Their strengths and weaknesses are complementary. Because IVOs are seen as legitimate, they are able to conduct on-site inspections to verify declared activities and to confirm or disprove allegations of clandestine cheating. 

James M. Acton
Acton holds the Jessica T. Mathews Chair and is co-director of the Nuclear Policy Program at the Carnegie Endowment for International Peace.
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NIAs are more flexible and have a greater ability to uncover preliminary evidence of clandestine activities on which further investigations can be based. Such investigations require NIAs to share intelligence with IVOs. While this kind of intelligence sharing is generally permitted by arms control agreements, it is controversial.

Nonetheless, it appears to have become more common in recent years, particularly during the International Atomic Energy Agency’s investigation of Iran’s nuclear program. While intelligence sharing creates risks for both IVOs and NIAs, it is ultimately critical to the effective verification of arms control agreements and steps can and should be taken to ensure it becomes more common and less controversial....

The chapter "International Verification and Intelligence" was originally published in the special issue "Intelligence and Nuclear Proliferation" of Intelligence and National Security