Recent tensions between the United States and its European allies have reignited questions about the legal boundaries of NATO membership. After US President Donald Trump accused allies of cowardice for refusing to help reopen the Strait of Hormuz—a waterway carrying a fifth of global oil supplies, currently disrupted by Iranian actions—a leaked Pentagon email suggested Washington might consider suspending Spain from the alliance. Madrid had denied the US access to its military bases and accused the Trump administration of violating international law in its strikes on Iran alongside Israel.
But can a member state actually be suspended or expelled from NATO? The short answer is no. The alliance's founding treaty, signed in Washington in 1949, contains no provision for suspension or expulsion. In nearly 77 years, internal disputes have been rare; the most notable followed Turkey's 1974 invasion of Cyprus, which raised concerns but involved a non-member. The leaked email's suggestion is therefore legally impossible.
What Article 5 Does—and Doesn't—Cover
Article 5 is the cornerstone of NATO, enshrining the principle that an attack on one member is an attack on all. It obliges allies to assist any member subjected to an armed attack, but only those of an international nature originating outside the alliance. Domestic terrorism is not covered. The clause has geographical limits: it applies to attacks on the territory of members in Europe and North America, plus Turkey and islands under their jurisdiction in the North Atlantic area. It also covers members' armed forces, vessels, and aircraft operating in or over these zones. As of 2024, NATO comprises 32 countries, including Finland and Sweden, representing roughly one billion people.
Article 5 has been invoked only once, after the September 11, 2001 attacks on the United States. That led to international involvement in Afghanistan, with allies such as Denmark, the United Kingdom, Germany, and France contributing forces. Denmark lost 44 soldiers in Afghanistan—proportionally more than any other ally except the US. France lost 90, the UK 457. Prince Harry, who served multiple tours, said those who gave their lives should be spoken about truthfully and with respect. The military response was US-led; NATO later took on support roles through the International Security Assistance Force (ISAF) and the Resolute Support Mission, which included non-NATO countries like Jordan, Australia, and South Korea.
No Obligation in the Strait of Hormuz
NATO is a defensive alliance and is not obliged to support military actions initiated by members outside its territory. Allies were not formally consulted ahead of US strikes on 28 February, and the Strait of Hormuz is not covered by Article 5. The war in Iran has indirectly affected NATO: air defences intercepted Iranian ballistic missiles entering Turkish airspace, F-35 jets were diverted from exercises, and a training mission in Iraq was suspended. Trump has called on allies to help secure the strait, but the alliance has no legal requirement to act.
This episode underscores a broader reality: NATO's strength lies in consensus, not compulsion. While political pressure can strain relationships—as seen in tensions between Copenhagen and Washington over Greenland, or between Paris, London, and Washington over Trump's comments about non-US troops—the legal framework remains unchanged. For now, no member can be suspended, and the alliance's collective defence clause remains a carefully defined commitment, not a blank cheque.
