The UN Charter does recognise two exceptions to this general probation. States may use force in self-defence (Article 51), and the UN Security Council may authorise the use of force for the purpose of protecting international peace and security (Chapter VII).

^{An Isil Command and Control Centre in Syria before, left, and after it was struck by bombs dropped by a US F-22 fighter jet (US Department of Defense)}

How does the law apply?

On 14 August, the UN Security Council passed resolution 2170 deploring the terrorist acts of Isil, and “its continued gross, systematic and widespread abuses of human rights.” Acting under Chapter VII, it urged all states to protect the civilian population and to cooperate in bringing Isil to justice. However, the key phrase, to use “all necessary means”, which in UN-speak is code for use of force, is missing.

The closest the resolution comes to this is when it calls upon “all states to take all measures as may be necessary and appropriate in accordance with their obligations under international law to counter incitement of terrorist acts.” Lawyers have not read this as UN authorisation to start bombing Isil. This, then, leaves the legal right of self-defence.

^{Fighters from Isil during a parade in Raqqa, Syria (AP)}

Usually self-defence applies when a state has been attacked, and must use force to repel the attacker or prevent further attacks. Thus, the United States clearly exercised a legal right of self-defence when it used force against al Qaeda and their Taliban hosts in Afghanistan following 9/11. However, Islamic State has not attacked the United States. There is a right of anticipatory self-defence under customary international law, but here things get a bit murky.

Most international lawyers accept that states may use force pre-emptively against an immediate threat, for example, to strike first where there is clear evidence that an opponent is about to attack. Under the Bush administration, the United States claimed a right, exclusively to itself, to use force to prevent a threat from emerging.

This could be applied here, in that it might be argued that Isil presents a latent threat to the United States, with the potential for launching al Qaeda style attacks against US facilities in the region if not the US homeland. However, most international lawyers reject the notion that there is a right of preventive self-defence, recognising the danger that it simply gives licence for powerful states to use force aggressively.

^{Blast as US air strikes hit Syria}

In the name of domestic security

States are permitted to use force to remove internal threats to their security, provided force is used in a manner that is consistent international humanitarian law, especially in terms of protecting civilian lives and property. States may further request military assistance from foreign powers, which may be legally given provided that the rebellion is not so widespread as to undermine the legitimacy of the government, as is the case in Syria. Isil has up to 30,000 fighters, and controls a large swathe of territory across Iraq and Syria (an area roughly the size of Jordan). However, ruling by fear, it lacks a widespread popular base of support, and therefore cannot be said to credibly challenge the legitimacy of the Iraqi state. The formation of a more representative Iraqi government under Prime Minister Haider al-Abadi, further undermines the legitimacy of the Isil insurgency.

Thus, since US air strikes against Islamic State targets in Iraq have occurred at the request of the Iraqi government, they are legal.

Syria and the United States see a common enemy in Isil. However, cooperation between the two is impossible. Following terrible war crimes by regime forces in the Syrian civil war, the United States no longer recognises the legitimacy of the Syrian government. The problem is that there is no legally recognised government to replace it. Thus, US air strikes have been conducted in Syria without a request from the national government. Thus, in bombing Islamic State targets in Syria, the United States cannot credibly claim that it used force in self-defence or at the request of the Syrian state exercising lawful force to suppress rebellion.

^{Isil militants celebrate as a vehicle drives on a newly cut road through the Syrian-Iraqi border (AFP)}

Illegal but justified?

Not for the first time, the United States has acted illegally in using force in response to overriding humanitarian necessity. It did so in March 1999, when along with its Nato allies it launched an extended bombing campaign to stop atrocities by Serbian forces against civilians in Kosovo. In this case also, the United States could not claim it was acting in self-defence. Nor was military action authorised by the UN Security Council. Whilst there was just cause, humanitarian necessity is not recognised in international law as constituting a legal ground for use of force. Thus, among the Nato allies, only Belgian claimed a legal right to use force for humanitarian reasons.

State opinion was divided following Nato's war in 1999. Many states, especially western, recognised the legitimacy of Nato's actions even if few recognised the legality. Russia and China attempted to pass a UN Security Council condemning the Nato bombing as illegal. A year later, in April 2000, the G77 group of 133 non-industrialised states issued a statement rejecting the “so-called right of humanitarian intervention.” Not much has changed since 1999. Indeed, if anything, attempts by the Bush administration to claim a right of preventive self-defence and fallout over the dubious legality of the 2003 Iraq War, have hardened most states’ views against accepting the legality of humanitarian wars.

There is an added strategic imperative, in that Isil military advances threaten the viability of the Iraqi state, in which the United States has much invested, and threaten the stability of the wider region. This is underlined by the involvement of five Arab states – Bahrain, Jordan, Qatar, Saudi Arabia and the United Arab Emirates – in the strikes against Isil in Syria.

The upshot is that US strikes against ISIL in Syria are probably illegal but widely recognised as legitimate. We are likely to see a rerun of what happened in 1999. 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.

Theo Farrell is Professor of War in the Modern World and Head of the Department of War Studies at King’s College London. He is co-author of International Law and International Relations (2012).