Article 39 of the UN Charter is the hinge on which the collective security system turns. Before the Security Council can authorise sanctions, force or any coercive measure, it must determine that a situation constitutes a “threat to the peace, breach of the peace or act of aggression”.
However, the charter never defines these terms. This silence is meant as strength, giving the Security Council flexibility to respond to unforeseen crises. Eight decades later, it has become a source of strain, since the power to define a threat is, in practice, inseparable from the interests of the five states holding a veto over any such determination.
Those who drafted the charter in 1945 envisioned Article 39 addressing the classic problem of interstate aggression, in which one state attacks or threatens another, but in practice it quickly moved beyond that model. Chapter 7 of the charter, practice broadened as conflict itself changed, extending to civil wars and state violence against a country’s own population once such situations carried cross-border security and humanitarian consequences.
Some landmark moments are well known: apartheid-era South Africa’s policies were found to threaten international peace and security; the wars in the former Yugoslavia met the same threshold; and the Rwandan atrocities, together with the collapse of order in Haiti, were treated as grounds for coercive action, including an international tribunal. Later scholarship describes this as an “increasingly dynamic, evolutionary” reading of the charter – case-by-case jurisprudence rather than a fixed doctrine.
That flexibility depends on consistency to remain credible, and consistency has proven difficult to sustain. The Syrian war illustrates this well. This was a conflict the included widespread killing, the use of chemical weapons, mass displacement and regional destabilisation comparable to that of Rwanda or the Balkans – yet the Security Council never agreed Syria met the Article 39 threshold.
The blockage was procedural rather than factual – veto powers meant a determination requiring consensus among states with divergent interests simply could not be reached. The episode reveals a structural feature of the system, not the failing of any single capital. A body meant to determine threats to peace on the basis of fact is, by design, also a body whose determinations depend on the alignment of interests among five veto-wielding states.
What was true of Syria has since recurred in three theatres, each showing how the Article 39 threshold interacts with the interests of veto powers rather than the facts of a crisis alone.
Since the war on Gaza began in October 2023, the Security Council has met on the situation dozens of times and witnessed at least eight vetoes cast on Gaza-related drafts. Only four resolutions have been adopted, each surviving only because one or more permanent members abstained rather than opposed it. None made a formal Article 39 determination, notwithstanding a very high death toll, severe humanitarian conditions, and an International Court of Justice advisory opinion addressing Israel’s obligations in the occupied Palestinian territories. Council members across the spectrum have acknowledged the gap between the crisis and the council’s capacity to act. This reflects the same dynamic seen in Syria – a determination requires an alignment of interests among veto-holders that this crisis has not produced.
The war in Ukraine is often described as the paradigm case of the interstate aggression that Article 39 was designed for – and it also most starkly exposes a design limitation of the Charter. When the state alleged to be responsible is itself a permanent Security Council member with a veto, then the council cannot formally characterise the situation as such over that member’s objection. The UN General Assembly, through Uniting for Peace-style resolutions, has therefore carried much of the burden of response, precisely because the Security Council’s own mechanism becomes structurally unavailable in this case.
The 2025–2026 escalation involving Iran, Israel and the US further illustrates how Security Council practice under Article 39 tracks the sequence and diplomatic alignment of events rather than a fixed standard applied uniformly to all parties. Following the initial strikes on Iranian facilities last year, and the wider war that began in February, the council convened repeatedly but reached differing outcomes at different stages, with no determination made regarding the strikes that opened the wider conflict.
Read together with Syria, Gaza and Ukraine, the pattern suggests determinations emerge most readily where they align with broad diplomatic consensus, and least readily where the facts touch the direct interests of permanent members or their partners – regardless of which states are involved. This is a systemic feature of a council built on great-power consensus, not evidence of bad faith by any single government.
The subsequent trajectory reinforces the same diagnosis. A June memorandum of understanding between the US and Iran, intended to end hostilities and restore transit through the Strait of Hormuz, broke down within weeks amid renewed attacks on shipping and a fresh round of strikes. Iran again restricted passage through the strait, and the US reinstated a naval blockade of Iranian ports, disrupting a corridor carrying roughly a quarter of the world’s seaborne oil trade.
The collapse of a bilateral understanding into renewed hostilities, entirely outside any Security Council framework, is itself another chapter in the deterioration of collective security. This is a crisis of direct relevance to international peace and the global economy has been managed, and then unmanaged, through ad hoc bilateral arrangements rather than the mechanism the Charter designed for this purpose.
Three consequences follow, and none are abstract. First, there is the erosion of charter authority as law rather than politics. If a threat to peace is whatever five permanent members can agree to call one, Article 39 functions less as a legal standard than as a voting outcome; this is a much less stable basis for order.
Secondly, there is an increasing recourse to unilateral and proxy action as substitutes for collective security. When the Security Council cannot reach consensus – as regards Syria, and increasingly in Gaza – states have tended to act through direct intervention, humanitarian justification or support for proxies, rather than wait for authorisation that may not come. The Iran-related escalation extends this logic: where paralysis is anticipated in advance, states may act first and treat bilateral arrangements, not the Security Council, as the primary mechanism for managing the aftermath.
Thirdly, there is pressure on the perceived legitimacy of the veto itself. Each veto is a legitimate power the UN Charter grants equally to all five permanent Security Council members. Cumulatively, however, the frequency of veto use across unrelated crises – irrespective of which member casts it – has raised questions across the UN’s wider membership about whether the founding bargain of permanent-member status, pairing special voting power with special responsibility, still functions as intended.
None of this means Article 39’s flexibility should be abolished. A rigid, fixed threshold would be just as easily circumvented and far less able to meet genuinely novel crises. The issue is not flexibility as such but the absence of any check on its selective use. Worthwhile proposals include voluntary veto restraint in cases of mass atrocity, of the kind France and Mexico have long championed; greater use of the General Assembly’s residual authority under Uniting for Peace when the Council is deadlocked; and a more disciplined, criteria-based approach to Article 39 determinations, making selective non-invocation harder to sustain even where it cannot be legally overridden.
Absent such reforms, the trajectory is difficult to mistake. Each unresolved crisis – Syria, then Gaza, then Ukraine, then the Iran-related escalation – makes the next instance of unilateral or bilateral action easier to justify, since the precedent of Security Council inaction is already set. The collective security system rests on the premise that the international community will name threats to peace consistently and respond to them collectively. What the past decade shows, across every case examined here, is not an absence of threats to international peace and security but an absence of the Article 39 determination that would trigger a collective response – a system whose findings track the diplomatic alignment of the moment more than the facts of the crisis, and an order adapting to that reality by increasingly working around the council rather than through it. That is not a footnote to the UN’s history. Instead, it is a signal of its declining centrality unless Security Council practice under Article 39 is reformed before the precedent hardens further.
