Donald Trump’s war in Iran and tirades against Nato allies are accelerating moves to develop a plan B for European security in case the US is no longer willing to help defend allies against a Russian attack. Europe must prepare for sudden vulnerability gaps if the fickle US president decides to pull out key military enablers before Europeans can develop their own alternatives.

European countries have already taken over financial and political responsibility for supporting Ukraine in its struggle against Vladimir Putin’s war of aggression, as Trump has increasingly sided with Moscow in trying to force Kyiv to hand over swathes of territory to Russia. After four years of war in Europe, most leaders have come to recognise Ukraine as a military and technological asset for European defence rather than a burden or a risk factor.

By branding Nato a “paper tiger” and European allies “cowards” for failing to support the US-Israeli war on Iran, Trump has undermined the credibility of the alliance. The announcement of a partial troop withdrawal from Germany as well as threats of further cuts and possible sanctions against European governments that withheld the use of their bases or airspace for Operation Epic Fury are forcing European leaders to think the unthinkable.

Several recent developments have highlighted how European countries are rethinking their security for a potentially post-American future. They can no longer be sure of US military backing if, for example, Putin were to stage a limited incursion into a Baltic state, perhaps sending covert forces to seize a Russian-speaking border town, and threaten massive, possibly nuclear retaliation if Nato responded. Such scenarios send shudders down the spines of European defence planners spooked by Trump’s unreliability.

In recent weeks, Germany has stepped beyond the horrors of its 20th-century history and issued its first military strategy since the second world war, setting the goal of becoming the strongest conventional army in Europe by 2039. France has opened talks with seven non-nuclear nations on extending its nuclear deterrent to cover European partners. That followed a UK-France declaration last year deepening cooperation between Europe’s two nuclear powers.

Emmanuel Macron has stressed that the French initiative is meant as a complement to US and Nato nuclear deterrence, not a substitute. Yet it is clear that non-nuclear countries such as Germany, Sweden, the Netherlands and Poland, which previously relied entirely on the American nuclear umbrella, are looking for new ways to deter Russia’s nuclear threat.

Although they are stepping up defence spending, European governments cannot in the short term replicate key capabilities provided by the US such as satellite intelligence, surveillance and reconnaissance, air and missile defence and aerial logistics. Nor do they have the command and control systems or logistics to mount a major military mobilisation without US engagement through Nato.

On Monday EU ambassadors held a first tabletop exercise to test how they would implement in practice their own hitherto largely declaratory mutual assistance pact. Article 42.7 of the EU treaty, which mandates “an obligation of aid and assistance by all the means in their power”, is on paper more binding than Nato’s better-known article 5. But this EU clause has only been invoked once – by France after the 2015 terrorist attacks – and triggered cross-border police and intelligence cooperation but not military action. The aim of the exercise was to establish procedures for defence emergencies. It reportedly did not factor in the possible role of Nato.

The mere fact of holding such a drill has sparked controversy, with Baltic states privately voicing concern that it could give Washington a pretext for disengaging from European security. However, the Polish prime minister, Donald Tusk broke a taboo by questioning publicly whether Trump would fulfil Washington’s Nato pledge in case of a Russian attack in the coming months. Europe’s “biggest, most important question is if the United States is ready to be as loyal as it is described in our treaties”, he told the Financial Times.

Europe’s rearmament drive also faces new challenges due to the Iran war. The conflict has massively depleted US arsenals of key weapons, and European governments are being notified that they may face years of additional delay before receiving vital air defence missiles, long-range fires and ammunition ordered from US defence contractors. This ought to give a boost to the European defence industry, except that fragmented European arms manufacturers are already at full stretch, with little or no capacity to meet increased demand. So Europe faces a lengthening window of vulnerability with less US support, political uncertainty and yawning capability gaps.

These dilemmas should be top of the agenda at the next Nato summit, in Ankara in July, but European leaders will be loth to air their strategic vulnerability with Trump for fear of prompting the impetuous US leader to walk out on them. Unsure how far they can still depend on the US, Europeans need to prepare for a gradual takeover of Nato’s tried and tested command structures, defence planning and joint exercises.

If possible, Nato should retain a US Supreme Allied Commander Europe (Saceur) to embody the enduring link with US nuclear and conventional military forces – a key deterrent to Russia. But Europeans must urgently seek ways to organise their own defence in case Trump shirks his commitment.

This is unlikely to be centred on the EU, given its unanimity principle and lack of military experience, and the fact that key allies such as the UK, Norway, Turkey and Canada are not members. If the US stands on the sidelines, a European defence union led by France, Germany and the UK, backed by Nato and EU resources, and with key roles for Poland and Ukraine, offers the strongest option to a vulnerable continent.

• Paul Taylor is a senior visiting fellow at the European Policy Centre