No, anticipatory self-defense against terrorist groups has not yet become an international customary law.

While some countries argue they have the right to strike first against a terrorist threat , this view is not universally accepted. International law generally allows self-defense only if an attack has already happened or is about to happen. There is still ongoing debate and no clear agreement among countries on whether anticipatory self-defense, especially against non-state actors like terrorist groups, is legal under international law.

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Laith Najeh Hameed

Anticipatory self-defense against terrorist groups has not yet achieved the status of international customary law, though it is a subject of significant debate and evolving practice in international law.

Background on Anticipatory Self-Defense

Anticipatory self-defense refers to a state using force in response to an imminent threat, rather than waiting for an armed attack to occur. The traditional understanding of self-defense, codified in Article 51 of the United Nations Charter, allows states to use force only in response to an armed attack. However, the concept of anticipatory self-defense, sometimes called preemptive self-defense, is more controversial.

Legal Debates and Evolution

The question of whether anticipatory self-defense is lawful has come under increased scrutiny, especially in the context of non-state actors like terrorist groups. Historically, international law, particularly since the Caroline Case in 1837, has permitted anticipatory self-defense only in the face of a threat that is "instant, overwhelming, and leaving no choice of means, and no moment for deliberation." This strict threshold makes it difficult to justify broad anticipatory action.

However, state practice has shifted in recent decades, particularly following the 9/11 attacks. Countries such as the United States and Israel have engaged in military operations against terrorist groups, citing the need to defend themselves against imminent or ongoing threats, even in the absence of a traditional armed attack. For instance:

• The 2001 U.S. invasion of Afghanistan following 9/11 was justified as self-defense against non-state actors (i.e., al-Qaeda), and the U.S. has since claimed anticipatory self-defense in operations against terrorist groups in other countries like Syria, Iraq, and Pakistan.
• Israel's operations against Hezbollah and Hamas have also been framed within this context, citing an imminent threat posed by these groups.

Does It Qualify as International Customary Law?

For anticipatory self-defense against terrorist groups to become customary international law, it would need:

While state practice is growing, especially among powerful states like the U.S. and Israel, many other countries and scholars challenge this interpretation, particularly because it risks undermining the UN Charter's prohibition on the use of force. The argument is that extending self-defense to situations where an attack is not imminent could erode the prohibition on aggression, which is a foundational principle of international law.

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