Inter­na­tion­al law differs in nature from domestic law in many ways.

In Australia, domestic law consists of Acts of par­lia­ment and judgments in court cases which interpret and apply those Acts.

In contrast, inter­na­tion­al law consists prin­cip­ally of:

1. treaties which are contracts entered into by States and are legally binding only on the States which are parties to them, and
2. customary law, which are rules that have been inferred from the way States have acted in practice and the opinions they have expressed about the legal effect of those practices. Customary law rules can therefore be hard to define, and they can change over time as the practices of States change, and as their opinions about the legality of those practices change.

In inter­na­tion­al law there is nothing equi­val­ent to an Act of par­lia­ment. The UN is not a par­lia­ment. It is a political forum that passes many res­ol­u­tions, the vast majority of which are not legally binding on member States. However, the terms of those res­ol­u­tions can help to shape new rules of customary inter­na­tion­al law if they are generally accepted by States without dissent.

The rules that apply to the resort to force by a State in self-defence against an external military threat were developed in the nine­teenth century, long before nuclear weapons were conceived let alone built. Under those rules, the external military threat has to be “imminent”. That is, the threat must be right on the point of devel­op­ing into an actual armed attack. Only then can the threatened State take military action to defend itself.

Applied strictly, this would mean that Israel could not take military action against Iran’s nuclear program until Iran actually develops a nuclear weapon and the means to deliver it. In the meantime, Israel would simply have to live with the threat of an Iranian regime that has openly and repeatedly committed itself to Israel’s destruc­tion and has illegally enriched uranium to levels of purity that go far beyond what is needed for any peaceful purpose.

No State would be prepared to live like that, under an ever-darkening shadow of anni­hil­a­tion. That is why there has been wide­spread accept­ance by most other States, including Australia, of Israel’s strikes against Iran’s nuclear program. That accept­ance, rein­forced by the US’s own strikes against Iran’s nuclear facil­it­ies, con­sti­tutes new State practice and legal opinion which is informing the devel­op­ment of new rules of customary inter­na­tion­al law con­cern­ing a State’s right to resort to military force in self-defence against an external nuclear threat.

So the short answer to the question of whether Israel’s military action against Iran is legal is yes.