Spying is known as the “great game,” but it can be rough. Despite many clandestine services’ official positions, spying has always been a game of “cloak and dagger”: the cloak covers and the dagger kills. Skills that let a spy get close enough to gain information are similar to those needed to get close enough to kill. The United States, United Kingdom, and Russia are no strangers to such work. All readily resorted to the tactic during the Second World War and Cold War. Even today antagonisms persist, as recent events demonstrate.

On March 4, Sergei Skripal and his daughter Yulia were found slumped over on a bench in Salisbury, England. Both had been poisoned by a nerve agent so deadly that the first policeman who arrived at the scene had to be hospitalized. All three were still in the hospital as of Monday, with Sergei and Yulia in critical condition. Sergei Skripal was a former Russian agent convicted of passing state secrets to MI6, the UK’s foreign intelligence service. He was later released as part of a spy swap with the US. Many believe Russian agents carried out the assassination attempt. On Monday, Prime Minister Theresa May said Russia was “highly likely” responsible for the attack, but Moscow has denied all responsibility. Amber Rudd, UK’s home secretary, has claimed that the assassination attempt is a “brazen act of war” and a “reckless crime” aimed at “humiliating the UK.”

Assuming Russians carried out the plot, what would be the UK’s appropriate response? Would it be just for the UK to target Russian spies in order to signal that Russians in the UK are off-limits and under the Crown’s protection? After all, the UK has had its sovereignty challenged when a foreign government sponsored violent acts within its boundaries.

Before Britain responds in any way, officials must be convinced that the government of Russia, rather than merely Russian agents, is responsible for the attack. Moscow’s denial is not entirely without plausibility. Russia had already caught Skripal and could have executed him then. It did not have to give him up in a spy swap. One could argue that, by attempting to assassinate Skripal, Russia makes it more difficult to conduct spy swaps with other countries in the future.

Moreover, as a double agent Skripal may have betrayed other Russian agents, and they could have decided to seek revenge without authority from their superiors. Or perhaps some “eager beaver” Russian agents wanted to make a spectacular statement, again without proper authority. Such things happen even within the UK’s spy services. For instance, in January 1975 a British intelligence team led by Captain John Ball and Lieutenant Robert Nairacn assassinated Irish Republican Army (IRA) commander John Francis Green. The agents had contacted intelligence officers in Armagh, Northern Ireland, told them they were under orders from MI6, and asked for suitable targets. Ball and Nairac were probably not under orders from MI6, which meant the assassination lacked proper authority. The assassination attempt on Skripal may also have lacked proper authority. True, this is highly unlikely, given how Russia disciplines its agents, but not impossible. Nevertheless, the matter must be sifted to some degree.

Assuming the Russian government can reasonably be shown to be guilty, can targeting Russian agents meet the demands of just war criteria? Right authority is presumably no problem if the prime minister agrees to the action. The just cause would be that the government is responding to an act of unjust aggression within its own borders. The UK’s very sovereignty and provision of a safe rule of law for its citizens are at stake.

In order to follow the just war principle of right intention, London must be able to show it intends good results and the operation will bring about those results. Just war tradition would also require that the UK’s response is efficient, necessary, and likely to produce more good than harm. The government should also show how the operation’s planning and execution had considered the consequences.

In the recent Skripal case, a British retaliation could lead to further Russian retaliation. Does the UK want to get into a quid pro quo assassination game? A cursory glance at the violence in Ireland and the Middle East in the twentieth century (and beyond) shows how hard it is to win that kind of game.

It would be an ideal and much better alternative if the UK could capture the Russian agents actually guilty of the crime. The opportunity to interrogate and then put them on trial would be a wonderful way to show how Britain, unlike Russia, adheres to the law of nations. However, barring an amazing intelligence coup, the UK is unlikely to uncover who actually did the poisoning. If one cannot capture the actual criminal, can one retaliate against the gang in order to deter future indiscretions on British soil? This is a matter of discrimination.

Enemy agents are certainly acceptable targets. Double agents in particular freely enter a game in which they know they are likely to be killed upon exposure. The testimony of the US Senate Church Committee, set up in 1975 to investigate CIA tactics, revealed that spy agencies expect “defrocked” double agents to be killed. Again, ideally, in the Skripal case not any Russian agent would do. We would like to find out who exactly is responsible, for this is a criminal act as well as a war-like act.

However, we do not need to discover who exactly is responsible in order to take just and effective action. If Russian agents tried to kill Skripal and his daughter, Russian agents should pay. What Russia did (assuming it is responsible) is analogous to a conventional military attack. Once an unjust aggressor has used military force, we may strike back in self-defense where we think most effective.

Of course, having decided upon an operation against Russian agents, not every means is justifiable. We could not, for example, use the nerve agents the Kremlin employs, for they are inherently indiscriminate. No matter. There are many other ways to eliminate unjust agents, and most of them can be used in discriminate fashion.

The desire for an effective response to Russian aggression brings us to the just war principle of proportion, the idea that our actions should cause more good than evil. Proportion is very hard to meet in cases like this because it is difficult to tell if targeting Russian agents or trying some other response is better. The consequences may be, at best, increased reciprocal acts or, at worst, a more unsettled world with increased chances of major open warfare.

But not responding has consequences. Russia and other enemies could be emboldened to “bully” the UK and other liberal states even more. History tells us that failure to respond to an unjust aggressor likely leads to more aggression. Nevertheless, proportion demands we think first about peaceful ways to check unjust aggression, ways that might be just as effective as targeting Russian agents but less likely to lead to a killing match. If there are no other effective means, and if the response can be discriminate and proportionate, then certainly targeting Russian agents is a justifiable option. If the UK decided upon such a response to unjust Russian aggression, they would have decided to apply a notable lesson of military history: small-scale killing matches sometimes prevent larger ones.

Darrell Cole is Professor of Ethics at Drew University. He writes regularly on the ethics of war and is the author of Just War and the Ethics of Espionage (to read a review by David Shedd, who served as acting director of the Defense Intelligence Agency, click here).

Photo: Yulia and Sergei Skripal at the restaurant Zizzi in Salisbury, England.