Islamic State: are US air strikes in Syria breaking the law?

The US and its Arab allies launched air strikes on Islamic State strongholds in Syria for the first time this week, with the UK looking increasingly likely to join in. The scale of Tuesday's attack, which involved fighter bombers, drones and Tomahawk cruise missiles, was seen as a big escalation compared with the previous attacks on IS in Iraq. But with no mandate from the United Nations nor consent from the Syrian regime, was the round of new air strikes legal?

Why have the airstrikes in Syria raised legal concerns?

The UN Charter constitutes a general prohibition against the use of armed force, although there are exceptions. The fact that the Iraqi government invited the US to help defend itself against Islamic State means the airstrikes in Iraq are legal, says Theo Farrell, head of the department of war studies at King's College London.

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But the legalities are more complicated when it comes to Syria. The US has confirmed that it warned Bashar al-Assad's Syrian regime about the air strikes but did not request the regime's permission. Writing in the Daily Telegraph, Farrell says the United States cannot therefore credibly claim that it used force at the request of the Syrian state exercising lawful force to suppress rebellion. Syria, Russia and Iran have all claimed that, without an invitation from Syria or authorisation from the UN Security Council, the air strikes have no legal standing.

What is America's defence?

US government lawyers have pointed to Iraq's right to self-defence and the weakness of the Assad regime as twin justifications for its airstrikes in Syria, reports The Guardian. In a letter to the UN, US ambassador Samantha Power argued that the threat to Iraq from Islamic State gave the US and its allies an automatic right to attack on its behalf under Article 51 of the UN Charter.

"Iraq has made clear that it is facing a serious threat of continuing attacks from Isil [Islamic State] coming out of safe havens in Syria," Power wrote. "The government of Iraq has asked that the United States lead international efforts to strike Isil sites and military strongholds in Syria in order to end the continuing attacks on Iraq, to protect Iraqi citizens and ultimately to enable and arm Iraqi forces to perform their task of regaining control of the Iraqi borders."

Following the war crimes committed by Assad's forces in Syria, the US and UK no longer recognise the legitimacy of the Syrian government, although The Guardian says this view remains "legally dubious".

Marc Weller, a professor of international law at the University of Cambridge, says the Syrian government is under obligation to secure its population from crimes against humanity committed on its territory, but is unable to do so after losing control over areas occupied by IS. Writing for the BBC, Weller argues that because Syria has been "unable or unwilling" to prevent IS operations against Iraq from its own soil, the zone of operations to defeat IS can be extended to cover portions of Syria beyond the control of the Syrian government under the UN doctrine of self-defence.

What about domestic law?

Obama's "legal acrobatics" to avoid requesting a UN Security Council mandate match similar efforts to avoid requesting specific legal authority from Congress, says the Guardian. "Fearing that US politicians up for re-election in November may balk at voting for a third military attack on Iraq and being sucked into a Syrian quagmire, the White House has avoided seeking a fresh authorisation of the use of military force, preferring to rely on early authorisations against al-Qaeda granted after the 11 September 2001 attacks," it says. However, the argument that Islamic State is equivalent to al-Qaeda has been questioned by several critics in Congress.

Are the air strikes justified?

The Times says Obama is right to ignore "querulous voices" suggesting the air strikes in Syria have a dubious legal basis. Even if it were not true that Islamic State represents an immediate threat to America and other countries, the "mass killings and the displacement of whole populations would constitute a sufficient practical and moral basis for air strikes and other military intervention", says the newspaper.

However, Farrell says that humanitarian necessity is not recognised in international law as a legal grounds for the use of force, even though the US and allies used it as a justification for launching an extended bombing campaign to stop atrocities by Serbian forces against civilians in Kosovo in March 1999. Farrell believes the US strikes against IS in Syria are "probably illegal but widely recognised as legitimate". He explains: "Some states may seek to reaffirm the illegality of using force for humanitarian ends or to otherwise interfere in the internal affairs of states. However, most states will welcome this necessary action and simply stay silent on the question of legality."