Who, What, Why: When is it legal to kill your own citizens?

Two British jihadists from the Islamic State group were killed by a RAF drone strike in Syria. What's the legal status of the attack?

Two UK citizens, Cardiff-born Reyaad Khan, 21, and Ruhul Amin, 26, from Aberdeen, were among three fighters from the Islamic State group killed in the precision strike in Raqqa on 21 August. The government says Khan was planning a series of attacks in the UK.

Prime Minister David Cameron said the attorney general had agreed there was a "clear legal basis" for targeting Khan. Defence Secretary Michael Fallon said the strike was a "perfectly legal act of self defence".

But concerns have been raised by senior politicians and human rights groups. Former attorney general Dominic Grieve has said the decision to launch the attack could be "legally reviewed or challenged", while Conservative MP David Davis said it would amount to an "extra-judicial execution" without formal checks.

So what is the legal status of the Syria operation?

Justifying its decision, the government said it was acting under Article 51 of the United Nations charter, which says member states have an "inherent right of self-defence". Cameron said there was "clear evidence" that armed attacks on the UK were being planned.

Article 51, Charter of the United Nations

Nothing in the present Charter shall impair the inherent right of individual or collective self-defence if an armed attack occurs against a Member of the United Nations, until the Security Council has taken measures necessary to maintain international peace and security. Measures taken by Members in the exercise of this right of self-defence shall be immediately reported to the Security Council and shall not in any way affect the authority and responsibility of the Security Council under the present Charter to take at any time such action as it deems necessary in order to maintain or restore international peace and security.

Writing in the Guardian, legal commentator Joshua Rozenberg argues that the attack "appears to be within the law". International law, like English law, doesn't require that you have to wait for an aggressor to strike before retaliating so long as the action is proportionate and necessary, he says. Under Article 51 you have to show that an armed attack is occurring or is imminent, said Phillipe Sands QC, professor of law at University College London.

The US used Article 51 to justify the killing of American-born Anwar al-Awlaki, a a radical American Muslim cleric of Yemeni descent who was linked to attacks and plots around the world, in a US drone strike in Yemen in 2011.

Following a failed attempt to blow up an airliner as it flew into Detroit in 2009, US President Barack Obama authorised the Central Intelligence Agency to kill Awlaki.

His family said he was not a terrorist and launched a legal challenge to stop the US executing one of its citizens without any judicial process. In a legal opinion published after Awlaki's killing, US Assistant Attorney General David Barron cited a 2006 Israeli Supreme Court decision that targeted killings were a legitimate form of self defence.

Sands added that the British government's use of the Article 51 line of argument represented a "new direction" for the UK, which had previously treated cases like this as matters for criminal rather than international law. Now, he said, the US "warlike paradigm" had been adopted instead.

"Planning a future attack at some far away place has never been good enough in international law on the use of self-defence - it has to be imminent and on that we need the evidence," he said. He added that the attorney general needed to make a statement to clarify the legality of the situation.

A ground control station at RAF Waddington, which operates drones

Labour and the Liberal Democrats have called on ministers to publish the legal advice, but Downing Street says it will not do so. Kat Craig, legal director of the human rights group Reprieve, said this meant the prime minister "has given himself a secret, unreviewable power" to kill anyone anywhere in the world at any time.

Some have argued that the government should have sought parliamentary approval for military action - especially as the House of Commons had previously voted against air strikes in Syria. But while there may be a convention that parliament should be consulted, it is not a legal requirement, Rozenberg says.

The issue is likely to continue to divide opinion - as it did during Northern Ireland's "Troubles", when British authorities were accused of operating a "shoot-to-kill" policy.

In 1995 the European Court of Human Rights found the shooting dead of three IRA members in Gibraltar was a breach of article two of the European Convention on Human Rights, the "right to life".

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