World leaders’ response to Donald Trump’s announcement that he would withdraw the US from the Paris agreement was strong and unified. But did it sting the president and his administration? To deter other potential backsliders and maintain the integrity of the Paris agreement, the perpetrator of a defection of this magnitude should be made to feel the pain. But how?

Trump – and other leaders that may later try to wiggle out of their commitments – must be shown that there will be real consequences to their credibility and influence and relations with other countries.

Ensuring the vitality of the agreement requires countries to show Trump that his ability to achieve other core diplomatic objectives will depend on honouring US commitments, and that he must consider more than the parochial interests of his most extreme supporters in determining how he will contribute to the global effort. If countries fail to offer a strong response, it risks setting the disastrous precedent that commitments can be treated as provisional, progression is not expected, and free-riding will not entail significant costs. This will only invite further defection.

In an important 2007 essay, founder of Bloomberg New Energy Finance Michael Liebreich proposed a strategy for countries to deter defection and achieve strong international climate outcomes. Liebreich argued that the best approach is for countries to be “nice, retaliatory, forgiving and clear”. That is, they should unilaterally offer to do as much as possible, retaliate against those who do not reciprocate, reward recalcitrant countries when they do step up, and let other countries know in advance that this is their strategy.

Since the US election, most countries have bent over backwards to be nice to Trump. Even as Trump wavered, more than 40 countries have joined the agreement and numerous others have reconfirmed their pledges. Few countries have called him out for his aggressive efforts to reverse Obama-era climate policies. Now that he has said he will withdraw, leaders have clearly expressed their dismay at this decision. But these words surely will not be enough to persuade him to change course, or to deter others from weakening their commitments. Where is the retaliation?

Many have suggested that border carbon adjustments on American goods might be an appropriate response. And for good reason: no country wants to expose its manufacturers to competition from countries that are free riding on its efforts. But since trade sanctions can be a blunt and self-defeating tool, countries should also seek leverage points in other areas of concern to the administration.

One obvious and immediate step would be to curtly decline his invitation to renegotiate the agreement, followed by an effort to isolate the US within the ongoing Paris negotiations. Because the Trump administration cannot leave the agreement until November, 2020, the administration could muck up the development of the “rulebook” that will guide its implementation. Arguably, with one foot out the door, the US may not retain much influence in these discussions.

But the US also negotiates in concert with the Umbrella Group, an informal bloc of countries that includes Japan, Canada, Russia, Australia, and New Zealand. By suspending collaboration with the US and declining to coordinate common positions, these countries could signal both their ongoing commitment to the agreement and their displeasure with Trump’s decision, while limiting the administration’s ability to make mischief in the rulebook negotiations.

Countries could also seek leverage outside of the climate policy space, focusing on issues of particular concern to the Trump administration. Consider Nato financing. As with the Paris agreement, Trump feels aggrieved that Nato is profoundly unfair because he believes that the US pays a disproportionate share of the costs. At the same time that he has refused to contribute the “billions and billions and billions” that he bizarrely claims the Paris agreement obliges the US to pay in climate assistance, he has demanded that the other Nato members shoulder more of the costs of the alliance.

Linking these two issues provides an opportunity to gain leverage. Nato members could reiterate their willingness to contribute more to collective security, but make clear that they view climate stability as critical to that security. They could then say that if the US will not address the climate dimensions of their common security challenges, they will need to pick up the slack, and that that may not leave much room for increased military expenditures. In fact, Angela Merkel has already made a similar argument with regard to Germany’s willingness to absorb a disproportionate share of the costs of ameliorating the ongoing refugee crisis.

Such cross-issue linkages are common in other areas of international relations, but have not been widely used to advance climate action. No doubt, it will not be easy for countries, even in coalition, to implement such an assertive climate diplomacy against the Trump administration. Confronting a bully never is. But because the precedent they will set in responding to Trump’s attack on the agreement may well determine whether the ambition mechanism can work as planned, nothing less than the integrity of the Paris agreement depends on it.