On 21 September 2013, gunmen stormed Nairobi’s Westgate mall, randomly firing at people and taking hostages. The siege lasted four days. At least 67 people are known to have died and more than 200 were injured. Somalia-based Al-Shabaab claimed responsibility for the attack. On 24 September 2013, while declaring the end of the siege, Kenyan President Uhuru Kenyatta promised full accountability for the attack.

After a four-day stand-off, security forces succeeded in ending the siege, killing five gunmen and apprehending several others. The question thus arises whether similar law enforcement action will suffice to ward off future attacks and address the overall threat al-Shabaab poses to Kenya and the region.

Based in Somalia, al-Shabaab is believed to have approximately 5000 fighters, with foreign jihadists reportedly traveling to the country to join the group. Despite having been forced out of most urban areas, al-Shabaab remains in control of most of southern and central Somalia. In February 2012, its leader, Ahmed Godane, pledged obedience to al-Qaeda head Ayman al-Zawahiri. Moreover, the group has been recruiting US and European citizens.

It is thus reasonable to assume that law enforcement alone will not eradicate the danger al-Shabaab poses. The Kenyan government may consider using military force against the group in exercise of its right to self-defence. This would not be the first time Kenya took defensive action against al-Shabaab. In October 2011, the Kenyan military entered Somali territory to fight the militants. In a letter to the Security Council, the Kenyan Permanent Representative reported that “Kenya, (…), has, after the latest direct attacks on Kenyan territory and the accompanying loss of life and kidnappings of Kenyans and foreign nationals by the Al-Shabaab terrorists, decided to undertake remedial and pre-emptive action”. 

After 9/11, the Security Council implicitly recognized that self-defence could be used against terrorist attacks. Since then, it is increasingly accepted that the right of self-defence, as enshrined in Article 51 of the UN Charter, can be invoked against non-state actors, as long as the customary law requirements of necessity and proportionality are met (see Chapter 10 of my book on this topic).

The question then arises whether the mall siege amounts to an armed attack, triggering the exercise of self-defence. The siege lasted four days, resulted in a high death toll, caused widespread injury and considerable property damage. Such attacks are generally accepted as reaching the threshold of armed attacks (see Chapter 3 of Tom Ruys’ book on this topic).

That consideration alone, however, is not enough to invoke the right of self-defence. The principle of necessity also requires that there is a sense of emergency (immediacy), which makes defensive use of force urgent and inevitable. If the threat can be neutralized through other means, the exercise of self-defence cannot be justified.

In this case, although the siege was ended through the action of security forces, the overall threat posed by al-Shabaab has not been diminished. In fact, on 25 September 2013, Ahmed Godane vowed to continue the group’s fight against the Kenyan government and threatened with a long-lasting war, more bloodshed, destruction and displacements. The next day, al-Shabaab fighters attacked two Kenyan towns near the Somali border, killing at least two people and injuring three others.

Al-Shabaab’s determination to continue the conflict poses a threat to Kenya and the region that may necessitate defensive use of force. The group has carried out attacks in the past and is determined to do the same in the future. The previous attacks, the mall siege, the attack on the border towns and the overt agenda of the group can create a security threat which renders self-defence necessary.

Such an action, however, must also meet the requirement of proportionality, in the sense that the defensive use of force should be aimed at neutralizing the source of these attacks and should not go beyond that purpose. In this case, self-defence would be exercised against the al-Shabaab group. This brings us to a question that goes to the essence of the law of self-defence: earlier this year, the US administration stated that it had a right of self-defence against “al-Qaida and associated forces” and suggested that “transnational non-state organizations such as al-Qaida” may have multiple bases for operations. The 2012 US Director of National Intelligence assessment described al-Qaida as a decentralized movement, with regional affiliates planning and attempting terrorist attacks and multiple voices providing inspiration for the movement. So what is al-Qaida? Is it a core with associated forces, a transnational organization or a decentralized movement? And how does the answer to such questions affect the right of self-defence? In particular, can attacks such as the one carried out by al-Shabaab be attributed to al-Qaida? After all, it has been repeatedly stated that al-Shabaab is linked to al-Qaida. Such questions should be treated with great caution. Clearly, more than one group can be held responsible for an armed attack, if it was jointly carried out. It is far from clear, however, whether sending members or providing weapons, equipment, training or funds to carry out a specific attack could be seen as basis for holding one group responsible for an armed attack carried out by another (so far, the rules on attributing private conduct have only concerned states). In any case, belief in a common purpose or ideology, encouragement, theoretical guidance, pledges of obedience or sporadic assistance between groups is not enough to render one responsible for an armed attack carried out by another. Neither can an ideological movement become the author of an armed attack.

Kenya has the right to defend itself against terrorist acts and the recent mall siege amounts to an armed attack that can justify self-defence against al-Shabaab. The fact that al-Shabaab has links to al-Qaida is important from a strategic, counterterrorist perspective, but that link cannot, in itself, expand the right of self-defence against other groups or individuals, even if they share the same ideology and pledge allegiance to the same movement.