Alliance, law and reluctance: Britain, the United States and the politics of joining other people’s wars

Thursday 5 March 2026

In March 2026 the question of whether the United Kingdom should join the United States and Israel in military operations against Iran has revived a familiar tension in Anglo-American relations. Washington has sought visible allied support, both for strategic reasons and for the symbolism of coalition warfare. London has hesitated, insisting that any participation must comply with international law and command domestic political legitimacy. The resulting friction has reminded historians and diplomats of an earlier episode: the pressure placed upon the British government in the 1960s to commit troops to the Vietnam War.

The comparison is instructive not because the two conflicts are identical but because the underlying dilemma is the same. The United States, possessing global military reach, often expects the diplomatic and occasionally the military support of her closest allies. The United Kingdom, despite the enduring mythology of the “special relationship”, must weigh such requests against domestic law, parliamentary authority and the structure of international legal obligations. In both the Vietnam War and the present confrontation with Iran, British leaders have been confronted by the same strategic question: how far can alliance solidarity extend before it collides with legal principle and political reality?

Three themes illuminate the comparison. The first is military strategy and the United States’ persistent desire for allied participation. The second is public opinion in Britain and the United States, which has repeatedly constrained governments. The third, and most revealing, concerns international law and the strikingly different ways in which the United Kingdom and the United States understand its authority.

Military strategy and the politics of coalition warfare

For the United States, coalition warfare has long served both operational and political purposes. Militarily, allies provide bases, logistical capacity and occasionally additional combat power. Politically, they confer legitimacy. American interventions that are visibly multinational appear less unilateral and therefore less imperial in character.

This was a central element of the Johnson administration’s thinking during the Vietnam War. Although the United States provided the overwhelming majority of troops, Washington sought contributions from allied states such as Australia, South Korea, Thailand and New Zealand. The United Kingdom was regarded as particularly valuable. As a permanent member of the United Nations Security Council and America’s closest diplomatic partner, British participation would have signalled that the war in Vietnam represented a collective Western effort to resist communist expansion.

Yet Prime Minister Harold Wilson resisted the request. Britain’s armed forces were already stretched by commitments east of Suez and by the defence of Europe within NATO. Deploying forces to Vietnam would have imposed military strain upon a country already experiencing severe economic pressures. But strategic considerations were not the only factor. Wilson also recognised that the legal and political justification for the war was fragile.

A similar dynamic can be seen in the current confrontation with Iran. American planners view allied participation not merely as military reinforcement but as confirmation that their campaign reflects international consensus. British bases in Cyprus and elsewhere in the Middle East possess significant operational importance, providing staging grounds for aircraft and logistical support.

Nevertheless the United Kingdom has approached the conflict with caution. Prime Minister Sir Keir Starmer’s government has emphasised defensive operations — protecting shipping in the Persian Gulf and safeguarding British personnel — rather than participation in offensive strikes against Iranian territory. From London’s perspective, the strategic benefits of joining the campaign are outweighed by the risks of escalation and the uncertain legal foundation for offensive operations.

Thus in both Vietnam and the present Middle Eastern crisis, American strategic logic has collided with British restraint.

Public opinion and the limits of alliance solidarity

Public opinion has played a decisive role in shaping British responses to American pressure. During the Vietnam War the British public gradually became sceptical of American involvement in Southeast Asia. Large demonstrations occurred in London during the late 1960s, particularly after the escalation of bombing campaigns against North Vietnam.

Wilson’s refusal to commit troops therefore reflected not only strategic prudence but also domestic politics. Sending British soldiers to fight in a controversial war would have been politically catastrophic for a Labour government already grappling with economic crisis.

The present situation exhibits a comparable pattern. British public opinion remains deeply influenced by the legacy of the Iraq War in 2003, when the United Kingdom joined a United States-led invasion on the basis of disputed intelligence concerning weapons of mass destruction. That episode created a lasting suspicion of Middle Eastern interventions justified by urgent security claims.

Polls conducted in early 2026 indicate that a large majority of British voters oppose direct participation in offensive military operations against Iran. The electorate’s scepticism has made it politically hazardous for any British government to commit forces without clear legal justification and parliamentary approval.

In the United States, by contrast, public opinion has historically granted presidents greater latitude in initiating military operations. Although American voters may later grow critical of prolonged wars (and there is already polling evidence that barely one quarter of the US population support the current US strikes upon Iran), the constitutional structure of the United States allows presidents to deploy military force rapidly without prior legislative authorisation.

These contrasting political cultures shape the way the two countries approach alliance commitments.

International law: two legal traditions

The most profound difference between the United Kingdom and the United States, however, lies in their respective legal systems and their relationship with international law.

In the United Kingdom, international law occupies a distinctive position within the domestic legal order. The British constitutional system is not based upon a single written constitution but upon a combination of statutes, common law principles and constitutional conventions. Within this framework, customary international law has traditionally been regarded as part of the common law unless contradicted by statute.

This principle, sometimes referred to as the doctrine of incorporation, means that rules recognised by the international community can become directly relevant in British courts. Judges have historically treated international law as persuasive authority when interpreting domestic statutes or assessing government conduct.

In addition the United Kingdom has integrated many international legal obligations into domestic legislation. The Human Rights Act 1998, for example, incorporated the European Convention on Human Rights into British law, allowing British courts to review government actions against international standards. Even after the United Kingdom’s departure from the European Union, this broader tradition of legal incorporation remains deeply embedded in British jurisprudence.

Consequently when British governments contemplate the use of military force abroad they must consider not only international legality but also the domestic legal consequences of violating it. Legal advisers within the government routinely produce detailed opinions assessing whether military action complies with the United Nations Charter, particularly the rules governing self-defence and Security Council authorisation.

The United States legal system approaches the matter differently. Although the United States constitution declares treaties to be part of the “supreme law of the land”, the American judiciary has historically distinguished between treaties that are self-executing and those that require implementing legislation. Customary international law is not automatically incorporated into American domestic law in the same way it is in the British system.

Moreover the United States constitution grants the president substantial authority as commander-in-chief of the armed forces. While Congress possesses the formal power to declare war, successive presidents have interpreted their constitutional authority broadly, conducting military operations without explicit declarations of war.

As a result international law occupies a more contested position in American political debate. Legal scholars and judges frequently disagree about whether international norms constrain presidential action. Some argue that international law forms part of federal common law; others insist that it binds the United States only when explicitly incorporated into domestic legislation.

The practical consequence is that American presidents often treat international legal constraints as matters of diplomatic prudence rather than binding legal obligations. British governments, by contrast, tend to treat them as integral to their constitutional responsibilities.

The United Nations Charter and the legality of force

These structural differences become particularly visible when considering the rules governing the use of force under the United Nations Charter. Article 2(4) prohibits the use of force against the territorial integrity or political independence of any state, except in cases of self-defence or when authorised by the Security Council.

British governments have generally insisted upon adherence to these rules. Legal advice provided to the government before the Iraq War famously revolved around whether existing Security Council resolutions could be interpreted as authorising military action. The controversy surrounding that interpretation reinforced the British political class’s sensitivity to international legality.

In the present crisis involving Iran, British legal advisers have reportedly emphasised the importance of demonstrating either a clear act of self-defence or an imminent threat before participating in offensive strikes. Without such justification, military action could violate both international law and the expectations of British constitutional practice.

The United States has historically adopted a more expansive interpretation of self-defence, particularly when confronting perceived threats to American forces or interests abroad. This broader interpretation reflects the strategic posture of a global military power but also the relative flexibility of American domestic law regarding international legal constraints.

Lessons from Vietnam and the present crisis

When Harold Wilson declined to send British troops to Vietnam, he did so despite considerable pressure from Washington. His decision reflected a calculation that British strategic interests, domestic politics and legal considerations did not justify direct involvement.

The same logic appears to influence contemporary British policy regarding Iran. Alliance with the United States remains central to British security policy, yet it does not automatically entail participation in every American military operation.

Indeed the history of the Vietnam War demonstrates that the special relationship can survive such disagreements. Britain supported American diplomacy and shared intelligence with Washington during that conflict even while refusing to deploy troops. The alliance endured because both sides ultimately recognised the limits of what could reasonably be expected from an ally.

Law, sovereignty and the future of the alliance

The present dispute over Iran therefore reflects more than a temporary policy disagreement. It illustrates the deeper constitutional and legal differences between two countries that nevertheless remain each other’s closest strategic partners.

In the United Kingdom, international law is interwoven with domestic law and parliamentary accountability. British governments therefore approach military intervention with a legal caution that reflects the country’s constitutional traditions.

In the United States, international law interacts with a constitutional system that grants the executive branch substantial autonomy in matters of war. American leaders therefore often perceive legal debates about military intervention as political questions rather than binding constraints.

These divergent legal cultures do not necessarily weaken the Anglo-American alliance. Instead they illustrate its complexity. The special relationship has never been one of simple obedience. It is a partnership between two states that share historical ties, intelligence cooperation and strategic interests, yet interpret law and power through different constitutional lenses.

In both the Vietnam War and the present confrontation with Iran, the United Kingdom has demonstrated that alliance loyalty does not require unquestioning participation in war. Rather it requires careful judgement about when the use of force serves both national interest and the principles of international law that Britain has long regarded as fundamental to the international order.