A version of this article first appeared on the European Council on Foreign Relations website.

The United States looks less lonely in its use of drone strikes against terrorist suspects than it did a week ago. In the last decade, targeted drone attacks against alleged terrorists under non-battlefield conditions have become a hallmark of the US global campaign against al-Qaeda and have attracted enormous criticism. While reluctant to publicly accuse the United States of acting illegally, EU member states have consistently avoided any official endorsement of US actions. A number of European countries have faced legal action and public controversy over revelations that their intelligence services provided information that was used by the United States for non-battlefield drone strikes, reflecting a widespread belief in Europe that the US drone campaign violates international law. But US isolation among its European allies ended in dramatic fashion this week, when British Prime Minister David Cameron told the House of Commons that the UK had carried out its own targeted attack on a British citizen in Syria.

Cameron’s announcement marks a landmark moment: For the first time, a European country has used military force to carry out the premeditated killing of a specific individual outside the context of regular hostilities. Nevertheless, Britain’s action, though a notable departure from the traditional European opposition to targeted killing, should not be seen as an isolated event. On the contrary, it is part of a broader trend whereby a number of European countries are increasingly looking towards military responses to terrorism overseas, of the sort that the United States has long employed.

Of course there is nothing new about the UK conducting drone strikes. British forces have used drone attacks for many years in Afghanistan, but these were part of a conventional military campaign against the Taliban and other insurgents. Troops from Britain and several other European countries were involved in fighting on the ground in Afghanistan; this was a clearly recognizable armed conflict with UN Security Council authorization and a more or less defined zone of hostilities. By contrast, during the years after September 11, 2001, European countries carefully resisted any public endorsement of the United States’ broader “war on terror,” a purported global armed conflict that was said to give US forces the right to target or detain al-Qaeda fighters as enemy combatants no matter where they were located.

In recent months, facing the rise of the Islamic State (IS) and the departure of hundreds of European citizens to join its ranks, the policy of some EU member states has begun to shift. In 2014, the UK, France, the Netherlands, Belgium, and Denmark began conducting air strikes in Iraq as part of an international coalition fighting against IS. The legal justification for the campaign was that coalition partners were meeting a request from the Iraqi government for help in protecting its security and its citizens against the threat of IS. Nevertheless the action appeared motivated at least in part by the ambition of suppressing an armed group that posed a direct threat to European citizens. In that sense, European attacks against IS in Iraq are arguably not fundamentally different from US strikes against al-Qaeda in Yemen, which have been (at times) conducted with the approval of the host government and which (again, at times) took place against the background of an armed conflict between al-Qaeda and Yemeni armed forces.

In the meantime, France has been engaged for over a year in a sprawling military operation in the Sahel that spreads across five countries and involves 3,000 troops. Operation Barkhane, as it is called, evolved out of French support for the government of Mali but has evolved into a much broader effort to suppress jihadism across a wide region. It uses drones to identify targets in remote parts of the Sahelian desert, though they are subsequently attacked through the use of assault helicopters and other methods. Operation Barkhane seems to fall into a grey area between an armed conflict and military counterterrorism efforts. Its commander recently suggested the operation should be expanded into further countries, including Libya and Nigeria. “It’s necessary to fight terrorists everywhere and at all times,” he has declared, in language that US President George Bush might have proudly owned.

The British drone strike in Syria is a further step towards US practice. In his statement to Parliament, Cameron made clear that the action was not part of a larger military campaign in Syria. Instead it was “a targeted strike to deal with a clear, credible and specific terrorist threat” to the UK. Reyaad Khan, the target of the strike, was said to be “involved in actively recruiting ISIL sympathisers and seeking to orchestrate specific and barbaric attacks against the West,” and there was “no way of preventing his planned attacks on [the UK] without taking direct action.” British Defence Secretary Michael Fallon subsequently made clear that other similar strikes may follow: “There are other terrorists involved in other plots that may come to fruition over the next few weeks and months and we wouldn’t hesitate to take similar action again,” he told the BBC.

Britain may not be alone among EU member states in using targeted anti-terrorist strikes for much longer. French President François Hollande announced on September 7 that France would begin surveillance flights over Syria and that it would be “ready to strike” according to the information it collected. Hollande linked the decision to the fact that attacks against France were being planned in Syria, and press reports went further in clarifying that any French airstrikes (France does not currently possess armed drones) would be intended to head off plots against French citizens. Not long ago, the Italian Foreign Minister Paolo Gentiloni also floated the idea of targeted strikes against terrorist groups in Libya, though there was no suggestion that Italy planned to carry out strikes itself.

It is worth noting that the legal basis for Britain’s drone strike in Syria (and presumably also for any future French attacks) remains different from the position asserted by the United States. Where the United States has consistently argued that it can kill al-Qaeda fighters as enemy forces in an armed conflict, the UK relies on a claim of self defense against a prospective threat. The difference is not merely academic: According to the logic of the US position, it can target al-Qaeda members even when the attack is not strictly necessary to prevent an incipient terrorist atrocity. However, in practice, President Obama has instituted guidelines for US drone strikes that impose similar requirements to Britain’s “self defense” justification. Indeed, targeting against individual threats has long appeared as a possible point of convergence between US and EU counterterrorism policies, as I argued in an ECFR policy brief two years ago.

Nevertheless, apart from the issue of whether targeted strikes are an effective or wise policy against terrorism, the UK’s latest move raises huge legal questions. First, Cameron’s statement seemed to blur together two separate points: whether it was lawful for the UK to use force on Syrian territory without that country’s consent, and whether it was lawful to deliberately kill the specific individual who was targeted. Without further clarification, it remains dangerously unclear what standards the UK is proposing as a basis for either aspect of its action (and the UK’s subsequent claim, in a letter to the UN Security Council, that it was also acting in the collective self defense of Iraq only muddied the picture further). Second, as a justification for the deliberate taking of life or the violation of another country’s sovereignty, lawyers have traditionally required proof that the action was necessary to prevent an imminent threat. It is impossible to know from what Cameron said whether that test was applied or met in this case. The episode raises the worrying possibility that online encouragement of terrorist action might be taken as equivalent to direct planning of an attack, and the UK government should clarify how it distinguishes between these two activities.

In the United States, President Obama’s guidance directed that US forces only target individuals where they presented a “continuing, imminent threat.” A cynic might be inclined to see that standard as an oblique way of depriving the notion of imminence of any substantive meaning. The larger point perhaps is that countries engaged in targeted killing urgently need to spell out the standards they are using and the way they are applied, if they do not want to fuel an erosion of the international rule of law and open the way toward widespread acceptance of states killing those they deem to be their enemies.

The shift toward military counterterrorism is by no means consistent among all European countries. On the contrary, it is only a minority who are involved in such action, and Britain and France are clearly at one extreme of the spectrum. Among the largest EU member states, there has been no indication that Germany has moved away from the opposition to targeted killing expressed recently by Defence Minister Ursula von der Leyen. However, the shift in European military practice should be the spur for urgent discussion across the continent aimed at elucidating the rules that should apply in such actions in order to preserve the tradition of European support for the international rule of law.