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The Comprehensive Nuclear-Test-Ban Treaty Organisation (CTBTO) grew from a long campaign to close the last legal gap in nuclear restraint: a global, verifiable ban on all nuclear explosions.
After partial limits in 1963 and regional bans later, negotiations at the Conference on Disarmament yielded a draft treaty that the U.N. General Assembly adopted in September 1996, opening for signatures on September 24. Because entry into force required 44 named Annex 2 states with nuclear expertise to ratify it, governments created a Preparatory Commission in November 1996 to build, test, and provisionally operate a verification regime before the treaty legally began. The result was the Vienna-based CTBTO Preparatory Commission, which today runs the system that would underpin a future treaty in force.
The CTBTO’s verification regime is the heart of its authority and its principal political capital. It rests on the International Monitoring System (IMS), which consists of 337 facilities combining seismic, hydroacoustic, infrasound, and radionuclide stations to continuously search for nuclear tests around the world. The International Data Centre (IDC) fuses and analyses these signals. While on-site inspections can be activated only once the treaty enters into force, the IMS and IDC already operate continuously, issuing bulletins to states that allow near-real-time screening of suspicious events anywhere on the planet. As of late 2024, the organisation had reported that more than 290 IMS stations were delivering data, including North Korea’s tests as well as thousands of earthquakes every year.
The CTBTO’s rise has been practical as well as legal. By building and running a global sensor network, it created a de facto verification regime, in turn helping stabilise the global moratorium on nuclear explosive testing that has held for all states (except India and Pakistan in 1998 and North Korea since 2006). It has also expanded the IMS’s scientific uses, including for tsunami warning and atmospheric research, strengthening the system’s constituency across governments and research communities and making abrupt abandonment more expensive.
However, the treaty itself remains in legal limbo. Most states have signed (187) and ratified (178) it, but its entering into force still hinges on the eight Annex 2 holdouts: China, Egypt, Iran, Israel, and the U.S. (signatories that have not ratified), and India, Pakistan, and North Korea (non-signatories). In 2023, Russia revoked its prior ratification while pledging to continue observing the testing moratorium, a move that undercut momentum and highlighted strategic mistrust even as Moscow kept engaging the CTBTO’s technical work.
Against this backdrop, U.S. President Donald Trump’s public threat last month to restart nuclear testing, framed as regaining “equal” footing with Russia and China, poses immediate and longer-term risks for the global test ban. Analysts have noted that any US return to explosive testing would invite reciprocal tests by other nuclear-armed states and complicate the CTBTO’s political mission.
This said, significant practical hurdles stand in the way of new US nuclear testing. Decades of reliance on the Stockpile Stewardship Program have rendered the Nevada site’s explosive testing infrastructure and workforce in need of substantial refurbishment. Even supporters of Mr. Trump’s ideas have conceded that preparing an underground test would take time and substantial funds (with Congress’ approval — unfathomable during the longest US government shutdown in history). If the US conducts an aboveground test, that would collide with the country’s standing obligations under the 1963 Partial Test Ban Treaty and incur political and terrible environmental costs.
The US itself remains a CTBT signatory and successive administrations have certified its nuclear stockpile without explosive tests, even as authoritative trackers such as SIPRI and the Federation of American Scientists have documented processes to modernise it. If Washington were to test, it would forfeit the leverage stemming from unrivalled historical test data and the stabilising effect of a widely adhered-to taboo. It would also reduce the odds of ever meeting the CTBT’s threshold for entering into force by hardening opposition among non-ratifiers and undermining pro-treaty coalitions.
For the CTBTO itself, the immediate impact of a renewed US test would be paradoxical. Technically, the system would perform as designed, detecting, locating, and characterising the event and disseminating data worldwide; politically, however, the same success would coexist with damage to the organisation’s raison d’être. Put another way, an explosive test by the treaty’s most visible non-ratifying signatory would signal that the moratorium is contingent, and feed arguments that it may never enter into force. Russia’s 2023 de-ratification has already eroded the treaty’s legal standing after all.
Taken together, the rise of the CTBTO (in a previous era) illustrated a practical way to build an institution: create the tools, prove they’re useful, and let them accrue political weight even without formal legal authority. Its current status on the other hand reflects its fragility: it has a near-complete global sensor network and it has erected a strong taboo against nuclear testing, yet its inchoate identity leaves it vulnerable to fresh political shocks.
Whether Mr. Trump’s threat to restart US testing will overcome three decades of norm-building and monitoring capacity will be determined as much by choices in Washington, Moscow, Beijing, and other capitals as by the CTBTO’s continued delivery of timely, credible data that leaves little ambiguity about any state’s actions. The organisation can’t compel ratification or prevent unilateral decisions, but its core of transparency remains perhaps the most concrete path to eventually legally closing the test ban regime.
Published – November 09, 2025 02:48 am IST
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CTBTO: Global watchtower
