W HY, A STRATEGIST from Mars might wonder, do Europeans doubt their ability to defend themselves against Russia without American help? The total GDP of NATO ’s European members is more than ten times that of Russia, which has an economy about the size of Spain’s. They spend three-and-a-half times as much on defence as Russia, which has lately had to cut its budget sharply because of a broader squeeze on its economy. True, Russia has 13 times as many nuclear warheads as western Europe has, but surely Britain and France, the two nuclear powers, have more than enough to deter an attack?

For decades Europeans did not need to worry about the Martian’s question, because America’s commitment to their defence was not in doubt. That has changed. “The times when we could unconditionally rely on others are past,” Angela Merkel told the European Parliament in November. She echoed the call of France’s president, Emmanuel Macron, for “a true European army”. In January the two leaders signed a treaty between France and Germany which includes a mutual-security pledge similar to NATO ’s Article 5 (as well as Article 42.7 of the European Union’s Lisbon treaty).

This is sensitive territory. Mr Macron’s talk of a European army, and of “strategic autonomy”, irritates Americans. It is only prudent for Europeans to start hedging their bets against over-reliance on America, but hedging can be costly, and they have to be careful lest the hedge become a wedge, as François Heisbourg of the Fondation pour la Recherche Stratégique, a think-tank, pithily puts it.

Still, Mr Trump’s ambivalence about allies is almost an invitation to think through the implications of an end to Pax Americana. Suppose one morning a tweet announces that the United States is leaving NATO . Under Article 13 of the alliance’s founding treaty, a country can cease to be a member one year after notifying the government of the United States. So, bizarrely, Mr Trump would be serving notice on himself. An optimistic version of what happens next, apart from howls of protest, is that Europe makes a concerted effort to organise its own defence. Call it Europe United.

The conventional wisdom on Europe’s ability to protect its interests may be too defeatist, suggests Kori Schake of the IISS . The middle powers, in which she includes countries like Britain, France, Italy and the Netherlands, have been talking themselves into “exquisite uselessness”, but they have impressive capabilities. And, she argues, “the high-end American way of war is not the only way of war.”

A pale shadow

Yet the Europeans would immediately face institutional hurdles. Compared with Russia’s top-down system, command and control is hard enough in consensus-bound NATO . It would be a bigger challenge for Europeans alone, especially if they did not inherit NATO ’s command structure. The EU may want to take the lead, but military thinking is not in its DNA . Besides, an EU -only alliance would be a pale shadow of NATO : after Brexit, non- EU countries will account for fully 80% of NATO defence spending.

There would be gaps in capabilities, too. How bad these were would depend on the mission, and how many operations were under way at the same time. The European-led interventions in Libya and Mali exposed dependence on America in vital areas such as air-to-air refuelling and intelligence, surveillance and reconnaissance. A detailed look at the sort of scenarios Europe might face would help to identify other gaps, and what it would take to fill them. Bastian Giegerich of the IISS , who is starting to work on such assessments, reckons that realistically the gap-filling could take 15 years or so. That is a long time for places like Poland and the Baltic countries that feel under threat. Fear and mistrust could quickly conspire to make narrow national interests trump efforts to maintain European unity. Hence a second, perhaps likelier, version of what might follow an American withdrawal: Europe Divided.

Jonathan Eyal of the Royal United Services Institute in London imagines a frenzy of activity, a cacophony of summits—and a renationalisation of defence strategies. Lots of countries would seek bilateral deals. In central Europe he would expect an alliance between Poland and Romania to guarantee the eastern border. The Russians and Chinese would not sit idly by, he says, but would play their own games with the Greeks, Hungarians and others.

It is these games of mistrust that the American security guarantee has largely helped to avoid. They could all too easily resurface. “Establishing a purely European defence”, warns Michael Rühle, a long-time NATO official, “would overwhelm the Europeans politically, financially and militarily.”

That is why a third way forward for Europe looks more attractive: what might be called Europe Upgraded. This would involve the Europeans doing a lot more to improve their capacity in defence, but in ways that would help persuade the Americans to stay in: less loose talk about a European army, more effort to develop capabilities currently lacking.

Europe Upgraded sounds like an easy option, but it is not. It would demand cash, creativity and care. A serious push to plug the gaps in Europe’s capabilities would not be cheap. European governments, especially the big ones, would have to find a way to sell far larger defence budgets to their voters.

As for creativity, the European Intervention Initiative, championed by Mr Macron and launched last year, is an example of the sort of innovation that could help. It is inclusive: its ten members include Finland, not part of NATO , and Britain, soon to be out of the EU . The aim is to foster a common strategic culture that will help Europe respond more nimbly to crises in its neighbourhood without calling for American help.

The care is needed to make sure a more robust Europe is seen as supporting NATO rather than undermining it. America is suspicious of any duplication of NATO ’s efforts, such as the creation of rival headquarters. And bigger spending on defence could trigger disputes over industrial protectionism, especially if broader trade rows between Europe and America rumble on.

Even as the allies grapple with different visions of the future, a nuclear elephant has entered the room. Last October America declared (without warning the Europeans) that it was leaving the INF treaty, claiming a blatant violation by Russia, and served formal notice in February. Russia has since responded by pulling out too, threatening to develop new missiles. To make matters worse, the New START treaty, which limits strategic nuclear warheads and has strong verification provisions, is up for renewal in 2021.

A new nuclear-arms race would be a nightmare for NATO . In Berlin, Claudia Major is “enormously worried” that arguments over INF could divide Germany, Europe and the transatlantic alliance. Radek Sikorski fears that Russia’s missiles will leave Europe “defenceless” if it lacks a proportionate response to a first use of nuclear weapons by Russia, giving the Russians time to get where they want to by using conventional forces.

NATO has been here before. In the 1980s concern that Russian SS -20 intermediate-range missiles would “decouple” the European allies from America led to a dual-track approach: pursuit of arms control along with deployment of American cruise and Pershing II missiles in several European countries. The deployment went ahead despite mass protests, but the INF treaty signed in 1987 resulted in their removal and a long period of relative nuclear calm.

America is keen to maintain alliance solidarity, and officials say there are no plans to deploy intermediate-range missiles. There are other tools in the kit to keep Europe coupled. These include submarine-launched nuclear cruise missiles, currently in development, and new low-yield warheads for existing Trident missiles. A strengthening of missile defences would ramp up tensions with Russia.

The abandonment of the INF treaty is the most urgent reason to ask questions about the nuclear future. But the broader doubts about the strength of America’s commitment to defend Europe are also stirring things up. Like it or not, for the first time this century Europeans are having to brace themselves for a serious debate about the role of nuclear weapons on their continent.

Taboos could tumble. In a paper last November for the Finnish Institute of International Affairs, a French expert, Bruno Tertrais, suggested a range of “realistic” scenarios for expanding French and British nuclear protection, with or without NATO . Maximilian Terhalle, of the University of Winchester, and Mr Heisbourg recently argued that France should extend its nuclear umbrella to its European partners, including Germany. They acknowledge that “great leadership skills” would be needed to win support for this at home while not “prompting the withdrawal of US nuclear weapons from Europe”.