Is necessity the same as self-defense?

No. Self-defense (UN Charter Article 51) responds to an armed attack by another state; necessity under ARSIWA Article 25 can be invoked even when no other state has acted wrongfully.

Can necessity be invoked against peremptory norms?

No. Article 26 ARSIWA bars reliance on any circumstance precluding wrongfulness to justify breaches of jus cogens norms such as the prohibitions on genocide, torture, or aggression.

Necessity Defense (State Responsibility)

Q: Does invoking necessity erase the duty to pay compensation?

Not necessarily. ARSIWA Article 27 explicitly preserves the question of compensation for material loss caused by the act, even when wrongfulness is precluded.

A narrow plea under customary international law allowing a state to justify breaching an obligation to safeguard an essential interest from grave and imminent peril.

In the law of state responsibility, necessity is one of the "circumstances precluding wrongfulness" codified in Article 25 of the International Law Commission's 2001 Articles on Responsibility of States for Internationally Wrongful Acts (ARSIWA). Unlike consent, self-defense, or countermeasures, necessity is not invoked against a wrongdoer; it is pleaded unilaterally by a state that admits its conduct breached an international obligation but argues the breach was the only way to protect an essential interest.

Article 25 is drafted in the negative, signaling its exceptional character. A state may invoke necessity only if the act:

is the only way to safeguard an essential interest against a grave and imminent peril; and
does not seriously impair an essential interest of the state(s) toward which the obligation exists, or of the international community as a whole.

Necessity is excluded where the obligation itself rules out the plea, or where the invoking state has contributed to the situation of necessity.

The doctrine has deep roots in the Caroline incident (1837) correspondence between Daniel Webster and Lord Ashburton, though that exchange is more often cited for self-defense. The ICJ examined necessity carefully in the Gabčíkovo–Nagymaros Project case (Hungary/Slovakia, 1997), accepting it as customary law but rejecting Hungary's plea regarding the Danube dam project because the ecological peril was neither sufficiently grave nor imminent and alternatives existed. ICSID tribunals have repeatedly addressed necessity in the Argentine financial crisis arbitrations (e.g., CMS v. Argentina, LG&E v. Argentina, Sempra v. Argentina), reaching divergent conclusions on whether the 2001–2002 emergency met the Article 25 threshold.

Even where successfully invoked, necessity only precludes wrongfulness for the duration of the peril; Article 27 ARSIWA preserves the question of compensation for material loss caused.