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Michael Byers holds the Canada Research Chair in Global Politics and International Law at the University of British Columbia.

I’m hopeful that an all-out war between Israel and Iran can be avoided – because of an incident that took place on the Niagara River, south of Lake Ontario, almost two centuries ago.

A privately owned ship, the Caroline, was being used to ferry men and arms across the river from the United States in support of the 1837 Upper Canada Rebellion. British soldiers rowed across the river late one night, untied the ship, lit it on fire, and set it adrift over Niagara Falls.

The prospect of a war with Britain caused alarm in Washington. Daniel Webster, the U.S. secretary of state, wrote to the British representative, Lord Ashburton. They agreed that Britain had simply exercised a right of self-defence, since its actions were both necessary and proportionate.

As time passed, other countries also found it useful to distinguish between acts of self-defence and all-out wars. The right of self-defence became part of customary international law.

Since the atrocities of Oct. 7, 2023, Israel has claimed the right of self-defence against Hamas, Hezbollah and Iran.

In Gaza, the criteria of necessity and proportionality have not been met. Israel has transformed the conflict with Hamas into an all-out war, and the situation is now within the realm, not of self-defence, but of “international humanitarian law.” Those rules – on targeting, the prevention of unnecessary suffering, and the protection of civilians – are being violated by both sides.

In Lebanon, however, Israel has shown restraint, targeting Hezbollah facilities used for rocket attacks while avoiding most civilian targets. Hezbollah, for its part, has only launched a fraction of its large arsenal of rockets at Israel.

Iran is a state sponsor of terrorism that provides weapons and financial support to Hamas, Hezbollah and the Houthis of northern Yemen. Israel’s strikes against Iranian targets have been focused on curtailing that support.

Most recently, Israel struck the Iranian embassy compound in Syria, killing the two Iranian generals most involved in supporting Hamas and Hezbollah.

Strikes on embassies are prohibited under international law. Yet the right of self-defence is an exception to the general prohibition on the use of force in international affairs.

If killing senior Iranian officials was a necessary and proportionate response to attacks from Hamas and Hezbollah, it could be a legitimate act of self-defence. Academic international lawyers will debate the question for decades, but there’s no doubt in my mind that this was the conclusion arrived at by international lawyers in the Israeli Defense Forces.

Iran, however, saw the attack on the embassy compound as something that demanded a response. And given the widely accepted inviolability of embassies under international law, its own claim of self-defence must be taken seriously.

Significantly, Iran sought to keep its response within the bounds of necessity and proportionality. It provided advance notice that an attack would be launched against Israel, and it sent a relatively small number of drones and missiles. It then declared that it had acted in “legitimate defence” and the “matter can be deemed concluded.”

Israel then exercised its own right of self-defence against the drones and missiles, and did so with considerable success – thanks in part to U.S., British, French and Jordanian air defence systems in the region.

At this point, U.S. President Joe Biden called on Israel to stop. Its response, self-evidently, had been both necessary and proportionate. Mr. Biden also made clear that the United States would not become involved in an all-out war.

This last declaration should be a powerful deterrent to further Israeli action. However, it remains to be seen whether Israeli Prime Minister Benjamin Netanyahu will actually be deterred.

There are warmongers in Israel’s governing coalition, and Israeli soldiers are running out of targets in Gaza. The final option there, a full-scale assault on Rafah, would cause tens of thousands of further civilian deaths, and commensurate international outrage.

A war with Iran might be Mr. Netanyahu’s last hope for remaining in power.

International law cannot prevent all conflicts, but it can sometimes limit their extent. The right of self-defence enables force to be used where necessary, without starting an all-out war. When states claim the right, they are also signalling their desire to avoid an escalation.

This remarkable rule might be our best hope for peace in the Middle East today. Its origins are part of Canadian history.

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