In a world where nuclear weapons could soon be in the hands of a rogue nation like Iran, an Israeli preemptive strike on Iran’s nuclear facilities would be fully justified. Despite its ban on aggressive war, Article 51 of the United Nations Charter clearly recognizes a state’s inherent right to self-defense. Thus, Israel has full authority to act unilaterally or collectively in its self-defense.

Yet Article 51 does not create the right to self-defense; it is an inherent right of all states under customary international law. Hence, determining when self-defense is appropriate lies, as it always has, with each state.

Article 51 allows Israel to use aggressive force against Iran’s nuclear program if an “armed attack” occurs. Its plain language is satisfied when one state has used armed force to attack another state. Under customary international law, a preemptive strike is also permitted when an armed attack is imminent.

Thus an Israeli attack against Iran’s nuclear facilities would be legitimate for two reasons.

First, Iran already is conducting armed attacks under the plain meaning of Article 51 through Islamist terrorist surrogates Hezbollah, Palestinian Islamic Jihad and Hamas, so a de facto state of war exists between Israel and Iran.

Second, even if one questions whether armed attacks have occurred (by discounting Iran’s use of surrogates), Iran’s development of weapons of mass destruction constitutes an imminent, existential threat to Israel.

Under customary international law, the Article 51 “armed attack” requirement that evokes a right of self-defense can occur when a state perceives that such an attack is “immediately impending and inevitable.” Thus, rather than waiting for an actual attack, a state may execute a preemptive strike on the hostile state.

Historically, two elements must be met to legitimize a preemptive strike: proportionality and necessity.

The necessity element is where attention is usually focused. The acting state must have exhausted all other alternatives of dealing with the problem, and the threat from the hostile state must be imminent.

The traditional definition for imminent is when there is “some outward act that initiates the attempt to harm such that the actual harm is close at hand.” Under this definition, Israel likely would be required to wait until nuclear warheads were attached to missiles and about to be launched. But by then it would be too late.

The concept of imminence must be flexible over time, and the definition of “imminent attack” must be adaptable to modern warfare, since the traditional definition fails to account for the lethality and danger of WMDs.

If Israel deems that Iran is actively preparing for a nuclear attack against the State of Israel (an opinion amply supported by Iran’s continuous bellicose threats) and that failure to act would put Israel in grave danger of being unable to prevent that attack, the threat against Israel would in fact be imminent. As such, Israel would be justified in making a preemptive strike against Iran’s nuclear facilities, as long as the attack were proportional and Israel had exhausted all other meaningful alternatives.

Robert Ash is senior counsel of the American Center for Law and Justice, based in Washington. He wrote this piece with Jay Sekulow, the group’s chief counsel.