As a candidate, Donald Trump stirred great anxiety among our European allies with comments that called the US commitment to Nato into question. As president, he will have to reassure these allies that America’s commitment to their security remains strong and unconditional. And, as allies, Europeans will have to demonstrate that they are as committed to their own defence as they expect the US to be.

During his run for office, Mr Trump raised profound questions about the strength and importance of Nato, which stood as the cornerstone of America’s engagement in Europe for nearly 70 years. Not only did he assert that the alliance was “obsolete”, but he indicated that as president he would fulfil America’s obligation to come to an ally’s defence only if they “fulfilled their obligation to us” — meaning the need to “pay their bills”.

While questions about the adequacy of defence spending by European allies have been an issue within the alliance almost from the beginning of its existence, never before had any presidential candidate, let alone a president himself, suggested that America’s security commitment was conditional on allies paying their bills. By questioning the continued relevance of Nato and then making America’s security commitment under Article 5 of the Atlantic treaty conditional, Mr Trump has taken the alliance into uncharted territory.

Security alliances are, by their nature, based on trust. After all, a commitment by one country to come to the defence of another in case of armed attack involves a commitment to go to war even without suffering an attack oneself. In the nuclear age, such a commitment could involve a willingness to use nuclear weapons — and risk nuclear retaliation — in defence of allies that, in Nato’s case, are an ocean away.

For more than half a century, the US has sought ways to demonstrate its commitment to the security of its European allies — by deploying its forces on their territory, establishing an integrated command structure for military operations that included the participation of officers from all allied nations and setting up a political governing body within the alliance that worked on the basis of consensus. These steps, as the British historian Sir Michael Howard wrote more than three decades ago, constituted reassurance of allies, which, in an alliance such as Nato, is at least as important as deterrence of enemies.

Reassurance is about building and maintaining trust among allies. One of the most urgent steps for President Trump to take upon assuming office is to rebuild that trust by reassuring allies that the US remains fully and unconditionally committed to fulfilling its treaty obligations, including the all-important obligation to come to an ally’s defence if it is attacked.

At the same time, our allies in Europe would do well to take the debate about Nato during the presidential campaign seriously and do much more to enhance the ability to defend themselves. Mr Trump’s concerns about European failures to invest sufficiently in defence are not his alone; they are widely shared among Americans of all political stripes, and have been for a very long time. I spent more than four years urging Europeans to spend more on defence as US ambassador to Nato during Barack Obama’s first term as president.

Shortly before stepping down as defence secretary under two presidents in 2011, Robert Gates warned of “the blunt reality” that “there will be dwindling appetite and patience in the US Congress — and in the American body politic writ large — to expend increasingly precious funds on behalf of nations that are apparently unwilling to devote the necessary resources or make the necessary changes to be serious and capable partners in their own defence”.

Frankly, Europe did not heed that warning. And, while the 2014 Russian annexation of Crimea and invasion of Ukraine brought the decade-long trend in cutting European defence spending to a halt, the reinvestment in defence capabilities has been far too slow and far too slight. Indeed, while Nato countries long ago committed to spend 2 per cent of their gross domestic product on defence, today only five allies do so, with the remaining 23 allies aiming to get there only by 2024.

Europe can and must do better when it comes to defence spending. Ours is a dangerous and uncertain world, and adequate defence capabilities are necessary to address these dangers and uncertainties. Reassurance within an alliance works both ways — for allies to come to each other’s defence as well as to have the capabilities to do so.

Earlier this year, Nato leaders meeting in Warsaw committed to convene again in Brussels in 2017. A summit meeting early in the new year will provide Mr Trump and his European counterparts a necessary opportunity to reassure each other of their commitment to collective defence.

Ivo Daalder is president of the Chicago Council on Global Affairs and former US permanent representative to Nato

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