The expiration of the New START agreement between the United States and Russia on 5 February marks the near-complete collapse of an arms control system that once made nuclear competition predictable, verifiable and contained.

The risk is not simply enlargement of nuclear arsenals but the diminishment of safeguards against escalation, with shorter warning times and reduced transparency around capability development. As treaty-based limits disappear, strategic stability will be increasingly shaped by uncertainty rather than shared and verifiable rules.

For much of the Cold War, stability between nuclear powers did not rest on mere trust or goodwill; it rested on agreed rules, verification mechanisms and reciprocal restraint. Arms control worked not because adversaries became friends but because both sides accepted limits on the most dangerous incentives of nuclear rivalry.

This structure mattered because it reduced uncertainty, preserved decision-making time, and constrained escalation even during periods of intense political hostility.

While broader nuclear agreements such as the Nuclear Non-Proliferation Treaty remain in force, they do not regulate force levels or deployment between major nuclear powers. Over time, the key treaties that once formed this stabilising architecture have been dismantled—and the consequences are becoming increasingly visible.

The foundation of strategic stability was laid in the early 1970s with the Anti-Ballistic Missile Treaty. This limited missile-defence systems, forcing both sides to accept that second-strike capability would remain a credible deterrent. In doing so, the treaty reduced incentives for pre-emption and helped preserve decision-making time during periods of crisis—a central safeguard against rushed nuclear use.

However, the US withdrew from the treaty in 2002 to develop missile-defence systems, weakening deterrence by undermining the credibility of second-strike capabilities. This altered deterrence calculus increases the risk of escalation during crises.

While the arrangement was in force, verification was critical to its function. The question was never whether violations might occur but whether they could be detected quickly and addressed. When breaches were identified, they could be confronted, and in some cases reversed, precisely because both sides valued the framework itself. ‘Trust but verify’ was not rhetorical decoration; it was a functional principle underpinning strategic stability.

This logic carried forward into later agreements. The Intermediate-Range Nuclear Forces Treaty, signed in the late 1980s, eliminated an entire class of land-based missiles that had significantly shortened warning times, reducing pressure on leaders by giving them more time to make careful existential decisions.

However, that treaty lapsed following the US withdrawal in 2019, with Russia suspending its obligations in 2025. Its collapse reopened the door to advanced missile systems that compress warning times, narrowing leaders’ decision-making windows and increasing the risk of escalation.

In 2010, New START extended the Cold War framework, placing limits on strategic nuclear forces, capping deployed delivery systems at 700, deployed warheads at 1,550, and total launchers at 800 on each side. It also preserved transparency through inspections and data exchanges, even as political relations deteriorated. These agreements did not end rivalry; they constrained it.

The breakdown of arms control did not occur because the system stopped functioning. It occurred because states chose to withdraw from it. As treaties were abandoned, strategic incentives have shifted.

The disappearance of verification mechanisms allowed the emergence of new weapons systems—not necessarily as acts of provocation but as rational responses to a world with fewer constraints and diminishing predictability. Such advanced capabilities, combined with the absence of credible verification mechanisms, can reduce warning times and constrain leaders’ decision-making in crises.

The erosion of the rules has steadily replaced shared structure with strategic guesswork. The weakening of this stabilising architecture has produced a security environment that is faster, more opaque and more dangerous.

Indeed, arms control was never perfect. Treaties were contested, bureaucracies resisted and violations occurred. Negotiations were often politically unpopular and frequently portrayed as weakness. Yet despite these flaws, the system succeeded at its core task: preventing crises from accelerating uncontrollably toward catastrophe.

The lesson of the Cold War is not that peace emerges from trust. It is that stability emerges from agreed structures. Arms control was not idealism; it was institutional design. It  recognised that in a nuclear world, security cannot be unilateral.

The erosion of arms control, furthered by the expiration of New START, removes the mechanisms that once bought time, visibility and restraint. The result is a more uncertain environment as the systems that once moderated rivalry no longer apply.