You won't find a definitive answer here to the question of whether Reyaad Khan, a British citizen said to have been planning terrorist attacks under the banner of the Islamic State, was killed legally.
For one thing, it's hard to judge whether or not the government's action satisfied the international law on military force in self-defence without knowing what intelligence it had about what Mr Khan was doing. The government is unlikely to release that intelligence, as it's secret.
In addition, international law—the rules about dealings between nations—isn't as clear-cut as the internal legal system of an individual country. Some commentators have given their informed opinions already, which we'll point to as useful guides to the legal issues in this case.
"A clear legal basis"
The Prime Minister, in his statement to the House of Commons yesterday, said that there was "a clear legal basis for action in international law". While the legal advice of the Attorney-General to the government will not be published, Mr Cameron said that it was based on the UK's "inherent right to self-defence".
That's the language used in Article 51 of the United Nations Charter, an international treaty, which allows for self-defence "if an armed attack occurs".
This was only restating a far older idea in international law. The concept of self-defence includes the right to respond to an imminent attack, not just an actual attack, according to the former Foreign Office legal advisor Daniel Bethlehem and experts convened by the think-tank Chatham House.
In the wake of the 9/11 attacks, UN Security Council resolutions were passed that referred to the right to self-defence in the context of terrorist strikes—not just the armed forces of a particular nation.
Limits on self-defence
It's widely understood in international law that any military force in self-defence must be both necessary and proportionate to the threat faced. As the barrister Carl Gardner acknowledges in his piece on the legalities of the killing, the fact that the government's intelligence remains secret means that it's open to anyone critical of the operation to speculate on whether Mr Khan's terrorism plans were substantial and well advanced, and whether this strike was the only way of preventing them.
Michael Clarke, Director General of the Royal United Services Institute, makes a similar point. He writes that "a generic self-defence case would be a bit of a stretch in these circumstances. [The Prime Minister] may have to show that there was some specific reason why Reyaad Khan posed an imminent threat to the UK".
A British citizen, on foreign soil
While media focus is understandably on the British nationality of Mr Khan, and one of the other alleged Islamic State members killed alongside him, a former government lawyer told us that this is "legally irrelevant". What matters is not a person's nationality, but whether or not "they take a direct part in hostilities".
He also says that the killing in claimed self-defence taking place on Syrian soil wouldn't violate international law, assuming the Syrian government was "unwilling or unable to assist in dealing with the threat"—although this particular doctrine is not universally accepted. Mr Gardner wrote at greater length about it last year.
The slippery slope
Some legal commentators, including Joshua Rozenberg (our legal advisor), think that the government may need to do more to explain when it thinks international law is on its side in a situation like this one.
Philippe Sands, an international lawyer and academic, says that we need to have an idea where the line will be drawn on the use of military force against people suspected of planning terrorist attacks.
This echoes the commentator David Allen Green's concern that "to invoke Article 51 of the Charter is to perhaps push 'self defence' beyond the limits of elasticity".
That elasticity exists because definitive answers to questions of international law are comparatively rare. Many of its principles are hotly debated by academics, and the constraints it imposes on governments are as much political as legal. That makes the government's own view of the law particularly important.
Update 9 September 2015
We altered our headline, originally reading "Law and the killing of Reyaad Khan", as it was (unintentionally) the same as the title of one of the pieces we discuss in this article.
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