Friday’s horrific attacks in Paris have now prompted retaliatory French airstrikes on Raqqa in Syria. Given the emotions that have been stirred up, it may seem unnecessary to inquire as to the legal justification for the airstrikes. But even though precise legal argumentation may not be at the forefront of French priorities in a time of crisis, this is actually a crucial juncture in the evolution of the law of self-defense, with consequences far beyond this moment or this conflict. France has a real opportunity to lead here, and to ensure that the doctrine of self-defense does not get stretched beyond recognition — with potentially troubling consequences down the road should the opportunity be missed.

When French officials first announced on September 7 that France had begun to prepare for a possible bombing campaign in Syria, they did not immediately advance a clear legal rationale for the initiative. Whereas French policy had previously limited the scope of airstrikes to Iraq, political actors began to speak broadly about the need to attack Syria as part of France’s self-defense. And when President François Hollande reported the first strikes on ISIS training camps in Syria several weeks later, he stated vaguely that French forces were continuing to “identify[] targets that are training camps or places where we know that the Daesh terrorist group can threaten the security of our country.” Similarly, Prime Minister Manuel Valls asserted that “we are … acting in self-defense, which Article 51 of the United Nations Charter permits us to do.”

But of course self-defense needs to be more carefully defined, so the doctrine does not simply become open-ended. What sort of self-defense rationale was France using? France’s very brief letter to the United Nations Security Council discussing its initial Syrian military air campaign, made public only subsequently, gestured toward a variety of rationales. For example, although the letter invoked Article 51 of the UN Charter, it was unclear whether France was grounding its actions in collective or unilateral self-defense. On the one hand, the document emphasized that Iraqi authorities had requested assistance from the international community “in order to counter the attacks perpetrated by ISIL,” suggesting that France was intervening against ISIS to aid Iraq on a theory of collective self-defense. But the letter also referred to the terrorist acts by ISIS as a “direct and extraordinary threat to the security of France,” thereby invoking a unilateral self-defense theory. Further muddying the waters, the letter emphasized the “abuses committed against the civilian populations of the Syrian Arab Republic and Iraq, as a threat to international peace and security,” which might conceivably implicate yet a third rationale.

After last Friday’s shocking attack, the justification for unilateral self-defense is of course much stronger. In his speech at Versailles on Monday, President Hollande stressed that the Paris attacks were planned in ISIS-controlled parts of Syria and also indicated that further attacks could be imminent. These statements could provide the beginnings of such a theory.

Yet we need a more careful, publicly articulated legal argument as to the scope of a unilateral self-defense justification. Would self-defense justify French attacks against ISIS strongholds outside Syria? Against other anti-French terrorist groups in Africa? Against al-Qaeda in Afghanistan on some kind of associated forces theory or an independent basis? Against ISIS sympathizers the world over? Although Anthony Dworkin’s article suggests that French officials may not conceive of their “war” so broadly, a public statement (and debate) regarding the legal scope of the self-defense rationale is necessary. Indeed, a more detailed delineation of how far France conceives the self-defense justification to extend is all the more important if France wants to ensure its actions are not misinterpreted, as well as to help shape the doctrine of self-defense moving forward. It could also ensure that these attacks do not set a precedent for broad, amorphous political invocations of self-defense in other contexts by other countries. Russia’s deployment of the self-defense rationale for annexing the Ukraine is just one example that comes to mind.

As to the form such a nuanced argument might take, France might consider a new, more extensive letter of justification addressed to the Security Council. The September letter does not quite do the job. And of course the facts on the ground have shifted significantly. Thus, the reference to unilateral self-defense in the earlier letter, which at the time seemed ungrounded, could be explained in a more focused way. A new letter could articulate, for example, the specific credible threats of imminent attack that existed even on the earlier date. Moreover, while the use of the doctrine of unilateral defense to justify a military response against a non-state terrorist group is less controversial after the United States’ response to the 9/11 attacks, France has the opportunity to help delineate the contours of the circumstances in which such a response is appropriate. Although it might not seem to be in France’s self-interest to articulate limits to the doctrine of self-defense right now, a world where multiple states can invoke self-defense broadly is not likely to be in France’s interest over the long term. And France has the world’s ear right now. Later on, France will be just one of many voices on the topic.

A Security Council resolution affirming France’s response as an act of self-defense would help in this regard, and France has indicated it will seek such a resolution. Nevertheless, the Council would need to surmount Russian and Chinese objections, and significant differences are likely to arise over any resolution’s geographic scope.

None of this is to say that France is acting in a lawless way by retaliating in Syria. But it is important that the passions of the moment not be used to justify airstrikes anywhere, on any population, without real limits. We have already seen how a rationale advanced during the immediate aftermath of a terrorist attack — such as the 2001 Authorization for the Use of Military Force — can then be used for over a decade to justify a wide variety of activities in some cases far removed from the original context. Thus, we need to address the legal basis for action now.

This is a key opportunity to clarify what self-defense should mean in a time of global terrorism, and outline both when attacks are justified and when they are not.