Are Israel's airstrikes on Iran within legal bounds?

Is Israel's latest attack on Iran's military and nuclear facilities legal under international law? And would it be legal for the United States to intervene on Israel's behalf?
The answer to those questions gets to the heart of the most basic principles of international law, which draw on hundreds of years of precedents to lay out when countries can justifiably use force against each other.
Some experts say that if Israel is launching airstrikes on Iran solely to prevent a possible future attack, it would probably be illegal — and so would an effort by the United States to come to Israel's aid, as President Donald Trump considers whether to attack Iran's buried Fordo nuclear site.
Other experts argue that the current military operation is part of a continuing conflict that began when Iran's proxies attacked Israel in 2023. That could strengthen Israel's argument that its actions are part of the defensive measures that followed those prior attacks, and thus legal. That same argument would apply to the United States if it attacks Iran at Israel's request.
Jus ad Bellum and the Caroline Test
The rules governing when states can use military force are known as the law of jus ad bellum, or 'right to war.'
Jus ad bellum centers on the simple principle that states are prohibited from using force against each other, except in self-defense or if authorized by the UN Security Council. And even when the self-defense exception applies, the force must be limited to what is necessary and proportional. It is not a carte blanche for military conquest.
Although those principles are set forth in the UN Charter, the law behind them is far older. The Caroline test — a rule of customary international law that says states can use force only when absolutely necessary, to address an imminent, overwhelming threat — stems from 1837, when British forces crossed into the United States to destroy the American ship Caroline, to prevent rebels from attacking Canada. (Precedents in international law often involve ships.)
The principle still holds today that it is illegal to use military force to prevent a future attack that is not imminent.
Israel's current bombing campaign appears to fall afoul of that rule, some experts say.
'There is simply no plausible way of arguing that Iran was about to attack Israel with a nuclear weapon, which it doesn't even have,' Marko Milanovic, a law professor at Reading University in England, argued in a recent blog post.
In his speech announcing the military operation, Israeli Prime Minister Benjamin Netanyahu appeared to describe the country's actions as preemptive. He said Israel was acting 'to thwart a danger before it is fully materialized,' and that Iran had enough material to produce nuclear weapons 'within a few months.' Several days later, in a letter to the U.N. Security Council, the Israeli government said the operation 'aimed to neutralize the existential and imminent threat from Iran's nuclear weapon and ballistic missile programs.'
Iranian leaders have called for Israel's destruction in the past, and Israel's small size makes it especially vulnerable to nuclear strikes. However, US intelligence agencies assess that Iran has not yet decided whether to make a nuclear weapon.
Proxies and the Nicaragua Test
Other legal scholars see it differently, arguing that Israel's military operation in Iran is part of a defensive response to armed attacks by Iran and its proxies, including Hamas in the Gaza Strip, Hezbollah in Lebanon and the Houthi rebels in Yemen.
In that framing, Israel's attacks are not preventive, but rather part of an ongoing, justified self-defense operation.
'We are of the view that if the proxy war and the direct Israeli-Iranian hostilities are intertwined,' Amichai Cohen, a law professor at Ono Academic College in Israel, and Yuval Shany, a law professor at Hebrew University in Jerusalem, argued in a recent essay for the website Just Security, 'Israel is entitled to take self-defense measures against Iran, since some of its proxies — Hamas and the Houthis — continue to launch rockets against Israel almost on a daily basis with Iran's substantial involvement.'
For that to be true, Iran's influence over its proxies would need to meet a legal standard that is sometimes called the 'Nicaragua test,' which arose from a case involving the U.S. backing of the Contra militia in Nicaragua. If a state has 'effective control' over a militia, it can be held legally responsible for the militia's actions. And if it has 'substantial involvement' in a particular attack, it shares in the legal consequences of that attack too.
It appears unlikely that the 'effective control' standard would be met in this case, however. The members of Iran's so-called axis of resistance appear to have their own interests and to not be completely controlled by Iran. The New York Times has reported that Hamas failed to convince Iran to back its Oct. 7, 2023, attack on Israel, for example.
And while experts have long believed that Iran had considerable involvement in the military operations of Hezbollah, which began firing rockets on Israeli positions on Oct. 8, 2023, that group signed a ceasefire agreement with Israel last year, and for now appears to be staying out of the escalating conflict between Israel and Iran.
Iran does not appear to have effective control over the Houthis. However, the United States has accused Iran of being directly involved in the Houthi rebels' attacks on ships in the Red Sea, which began later in October 2023, by providing targeting assistance. And the Houthis' attacks on Israel are still going on.
Iran and Israel also traded direct strikes against each other's territory and personnel last year. In April, Iran fired hundreds of missiles at Israel in retaliation for an Israeli strike on an Iranian consular building in Damascus, Syria. Days later, Israel retaliated with strikes of its own against Iranian territory. Then, in October, Iran fired approximately 180 missiles at Israel in retaliation for Israel killing Hassan Nasrallah, the leader of Hezbollah, and Ismail Haniyeh, the leader of Hamas. But those strikes were relatively limited in both scope and time, so it is unlikely that they would be enough, on their own, to constitute an ongoing conflict.
And even if there were such a conflict, Israel's escalation would still need to be necessary and proportional to its defensive needs, Shany said.
What about the United States?
The legality of a possible US intervention in the conflict would most likely turn on the legality of Israel's actions, Shany said.
International law does allow collective self-defense, in which states provide assistance to victims of unlawful attacks, as long as the victim state requests it. That was why, for example, it was legal for the United States and other allies to assist Kuwait in repelling the Iraqi invasion in 1990.
But if Israel's actions are illegal, then the United States' participation in them would be too, unless there was an independent justification such as a separate need for self-defense against Iran.
International tribunals move slowly, so it is unlikely that Israel or the United States will answer for their decisions before a court soon, if ever. But the laws of war still matter.
The shared expectations they create are part of the foundations of the international order, helping to preserve peace and stability. The rules have never been perfectly followed, and the international order never perfectly peaceful or stable. But every time the rules are violated, those shared expectations weaken, making the world more uncertain and dangerous.

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