What is Nato’s central tenet?

Collective security, enshrined in the alliance’s article 5, which defines a military attack on one member as an attack on all. Such a scenario never arose in the cold war. The only time the collective premise was ever invoked was after the al-Qaida terror attacks on New York and Washington on 11 September 2001, when alliance members sent troops to Afghanistan in support of the US-led military effort.

Why is Trump considering withdrawal from the alliance?

Trump’s immediate ire has been piqued by Nato’s refusal to support or come to the US’s aid in the war against Iran. But nothing in Nato’s charter obliges its members to do so. The US was not attacked and did not consult fellow Nato states beforehand.

What has Trump recently said about Nato?

He called Nato “a paper tiger” and said withdrawing the US from membership was “beyond reconsideration” – words that implied his mind was made up.

He has also repeatedly said – citing the lack of support for the Iran war and European inaction to reopen the strait of Hormuz – that Nato would not protect the US, in the same way the US shields Europe. This is belied by the Nato support for the long war in Afghanistan.

Does the US president harbour deeper animosity towards Nato?

Apparently. In 2017, he dismissed the body as “obsolete” and accused its members, especially European countries, of “ripping off” the US by failing to spend adequately on their defence budgets. More recently, in 2024, he threatened to tell Russia to do “whatever the hell they want” to any European country that did not meet his defence spending demands.

Trump appeared on a collision with the alliance as recently as January, when the Nato members went on alert over his threat to annex Greenland. He backed down, but few believed his Greenland fixation had been permanently shelved.

Have Trump’s words had an effect?

Yes. After repeated browbeating, European leaders strove to keep Trump onside. Last June, Nato members agreed to raise their defence spending target to 5% of GDP by 2025. A decade ago, some countries were failing to meet a lower target figure of 2% of GDP. Moreover, Mark Rutte, Nato’s secretary general and a former Dutch prime minister, has gone to extraordinary lengths to flatter Trump – referring to him as the “daddy” of the alliance and earning a reputation as a “Trump whisperer”.

Rutte has even voiced support for the Iran war, in contrast to the opposition of Nato’s non-US members.

Does Trump have a point – is Nato a ‘paper tiger’?

Nato’s support for Ukraine – in the form of military help from both the US and European allies – has played a vital role in bogging down Russia’s invasion of the country, now in its fifth year. In doing so, it deters Russia from attacking actual Nato members (Ukraine is not a member of the alliance) such as Poland or the Baltic republics of Lithuania, Latvia and Estonia.

Vladimir Putin, Russia’s president, has made no secret of his desire to break up Nato or decouple it from the US – an outcome that could leave Russia’s eastern European neighbours open to future Kremlin aggression.

In the more distant past, Nato took military action in 1999 to deter the Serbian strongman Slobodan Milo?evi? from embarking on the ethnic cleansing of Muslims in Kosovo, a campaign that Moscow regarded as an affront and an incursion into its traditional sphere of influence.

In short, Nato – as it stands – is not a “paper tiger”. But its seems telling that Trump invokes Putin in describing it as such.

What does the US provide its Nato allies?

First and foremost, a nuclear umbrella. The US’s nuclear arsenal is vastly bigger than those of Britain and France. Additionally, there are numerous US military bases and installations throughout Europe, many of them centred in Germany. Another key site is Incirlik airbase in Turkey. All these bases are seen as providing deterrence against military attacks by the west’s enemies.

Could Trump simply withdraw such a vast panoply of armed backing?

It appears complicated. Legislation passed in 2024 prevents a US president withdrawing the US from Nato without a two-thirds Senate majority or an act of Congress. But Trump has shown himself willing to flout existing legislation to bypass Congress – for example, by attacking Iran without seeking congressional approval, as the 1973 War Powers Act says he must.

And there are hostile actions Trump could take short of outright withdrawal. Ivo Daalder, the Obama administration’s ambassador to Nato, has suggested a scenario in which Trump could withdraw all US troops and pull US officers from the command structure – all while saying he is within the terms of article 5, but not providing any military support.