US President Donald Trump revived a familiar refrain on Sunday (March 15), calling on allies in NATO to join the war commenced by the US and Israel against Iran, now in its third week.

In an interview with The Financial Times, Trump said NATO allies could face a “very bad future” if they refused to help the US in opening up the Strait of Hormuz. “It’s only appropriate that people who are the beneficiaries of the Strait will help to make sure that nothing bad happens there,” Trump said, arguing that Europe and China are heavily dependent on oil from the Gulf, unlike the US.

The latest remarks on the alliance have prompted a recall of Trump’s rhetoric at the start of the year, when he took to the stage at Davos to say that the US “never got anything out of NATO.”

Other NATO members (or allies) have struck a largely defiant tone: German Chancellor Friedrich Merz earlier said this war “is not a matter for NATO”, while NATO said “allies have already stepped up to provide additional security in the Mediterranean”. The leaders of the UK, Italy, Germany, Canada and France also issued a joint statement expressing “grave concern” over Israel’s invasion of Lebanon.

The US and Israel commenced their war against Iran on February 28, resulting in major retaliatory action that has engulfed the entire West Asian region. Part of this action has been the closure of the critical Strait, through which nearly 20% of global oil and fuel shipments flow.

Formed in 1949 with 12 member nations in the aftermath of World War II, the North Atlantic Treaty Organization or NATO is an intergovernmental alliance now comprising 32 members. Except for two nations – the US and Canada, all allies are European nations.

In marked contrast to most other alliances, NATO is defined by the commitment of its independent member states to secure mutual defence in the event of an attack by an outside party. Article 5 of the treaty governs this, enshrining an attack against one member as an attack against all members.

The parameters of the current war show a rejection of NATO’s rules of engagement under its charter.

Under Article 1 of the Charter, members must commit to resolve any international dispute they are involved in through peaceful means, without endangering international peace, security and justice. Allies are also expected to refrain from using the threat or use of force inconsistent with the purposes of the United Nations. The present conflict presents a clear rejection of this practice, with the fallout spreading globally.

Even the use of Article 5, obligating a response in the event of an attack on one member, may be invoked strictly as a defence measure, in response to an armed attack. Post 9/11, other allies were not obligated to join the US offensive in Afghanistan, but opted to join a Coalition of the Willing formed by the US, wherein individual allies pledged their support.

Allies may pledge monetary or humanitarian support in lieu of participating in military action, according to the Brennan Center for Justice’s interpretation of the Charter.

Geography is another limiting factor. Under Article 6 of NATO’s charter, the alliance’s collective defence obligations apply only to specific regions — broadly, the territories of member states in Europe and North America, Turkey, and the North Atlantic area north of the Tropic of Cancer, including their forces and vessels in the Mediterranean. The current conflict, playing out in and around Iran, falls squarely outside these boundaries.

Given these caveats, NATO exercises a high bar for military response. An attack on any of its military bases or the missiles belonging to member countries therefore, does not automatically invoke Article 5.

How things stand

Thus far, the alliance has limited its role to “enabling support”, such as logistics and missile defence, without invoking Article 5. Early into the war, NATO Secretary General Rutte emphasised that there are “absolutely no plans” for NATO to be involved, “other than individual allies doing what they can to enable what the Americans are doing together with Israel.”

Several European countries have rejected Trump’s call for naval reinforcements. German Defence Minister Boris Pistorius said that the country was looking for diplomatic solutions to the ongoing conflict. “This is not our war; we did not start it,” he said on Monday, adding that “sending more warships to the region will likely not help achieve that.”

Individual allies have chosen to engage at their own discretion. The UK has allowed the US to use some of its military bases in Cyprus following suspected Iranian drone attacks. While Cyprus is not a NATO member, the UK is. The UK has “thousands of service men and women in Cyprus”, three squadrons of fighter jets and counter-drone teams to help intercept Iranian attacks, British Prime Minister Keir Starmer said on Monday.

Other individual allies have also stepped up military deployments in the region. Greece has sent frigates and F-16s, while France deployed the frigate Languedoc to Cyprus. NATO forces have also intercepted Iranian drones and missiles near Turkey’s Incirlik Air Base. To date, the eastern flank of the alliance has absorbed the spillover of conflict, even as NATO itself refrains from formally entering the war.

Since his first presidential term, Donald Trump has claimed that other NATO allies haven’t contributed as much to the shared alliance defence budget as the US. Since 2006, each member has been expected to contribute at least 2% of its GDP on its defence, while a formal declaration in 2014 said that countries not meeting this goal would “aim to move towards the 2% guideline within a decade”.

According to NATO data, US defence spending constituted 63% of total defence spending in 2024, down from 72% in 2016, when Trump was first elected president. While both figures are substantial, the US ranks sixth in terms of the percentage of GDP spent on defence.

This January at Davos, Trump incorrectly claimed that despite the 2% stipulation, “most of the countries weren’t paying anything” until he came along. Total defence spending by non-US allies grew from $292 bn in 2016 to $482 bn in 2024. In 2024, 18 of 31 NATO members met the 2% defence spending target, up from four members in 2016 and eight in 2020. This increase in military spending was motivated in part by the war between Russia and Ukraine, which broke out in February 2022.

More importantly, Article 5 has been invoked only once, for America’s aid following the terrorist attack on September 11, 2001. Several NATO allies came together to stage a military intervention in Afghanistan. When the US exited it in 2021, there were about 10,000 NATO troops in the country (of which 2,500 were American), down from over 100,000 in 2011. Soldiers from Britain and other European nations died fighting in Iraq and Afghanistan.

Denmark alone sent 18,000 troops during this period, and had one of the highest per-capita death rates, losing 43 soldiers between 2002 and 2014.

Several European leaders have seen the 2003 invasion of Iraq as a costly mistake, driven by the faulty intelligence of then-US President, George W Bush.