When the US fired Tomahawk missiles at a Syrian airbase near Homs, it did so without warning and without permission.

It was a dramatic intervention triggered by a horrific gas attack on civilians, which the US says was carried out by the Syrian Government led by President Bashar al-Assad.

So, were the strikes illegal?

Short answer — yes, according to international law expert Professor Ben Saul from the University of Sydney.

Here's why:

'Retribution' isn't an excuse

When Malcolm Turnbull threw Australia's support behind the US, he said the strike had sent "a vitally important message, that we will not tolerate — the world will not tolerate — the use of chemical weapons".

"The retribution has been proportionate and it has been swift," he said.

Sorry, this video has expired Footage allegedly shows US missile strikes on Syrian air base.

But retribution isn't a good enough reason to fire a missile, Professor Saul says.

"International law very tightly regulates the use of military force, and using violence to punish another country is simply not permitted," he said.

When is military force allowed?

In two instances, according to Professor Saul — in self defence, or with the authorisation of the UN Security Council.

He says these are the only two circumstances in which the use of military force is legal under the UN Charter of 1945.

"Unfortunately, there's no further exception to the prohibition on the use of force for situations like stopping countries using chemical weapons, or preventing international crimes like ethnic cleansing of the kind we saw in Kosovo, for example, in 1999," he says.

(FWIW, international laws cover lots of elements of warfare. The five-minute video below from the International Committee of the Red Cross sums up some of the other rules.)

Loading...

And that doesn't apply here?

Well, Syria hasn't attacked another country.

And UN-sanctioned action seems unlikely, as Russia has used its Security Council veto seven times to protect the Assad Government, most recently in February.

"Taking this kind of action — even though it may be well-intentioned, even though it's designed for … good moral reasons — to stop this kind of atrocious use of chemical weapons against civilians in Syria, unfortunately is illegal under international law," Professor Saul says.

So, the Trump administration has broken the law. Is it in trouble?

Not really, Professor Saul says.

Why? Because the US is so powerful.

The US is one of five permanent Security Council members. ( AP: Alex Brandon )

"It obviously falls to institutions like the UN Security Council to deal with lawbreakers in the international community," Professor Saul says.

"If one of the lawbreakers is a great power, [one of the] permanent five members of the Security Council, then the chances of enforcement against a country like that are pretty slim."

"At this level, international law relies on countries doing the right thing, respecting their obligations in good faith under international law, and it also relies upon pressure being brought by other countries to call out law-breaking when it happens."

But so far, the US has a lot of international support

Germany, the UK, France, Japan, Turkey, Saudi Arabia and Qatar — to name a few — have all made statements supporting the Trump administration.

There haven't been many detractors, save Syria's allies Russia and Iran.

Iraq has criticised "hasty interventions" and Indonesia, which condemned the use of chemical weapons in Syria, said it was "concerned with unilateral actions by any parties".

"Military actions, undertaken without prior authorisation of the UN Security Council, are not in line with international legal principals in the peaceful settlement of disputes," the Indonesian Foreign Ministry said.

What should happen now?

Professor Saul says international law needs to adapt.

"If there are good reasons why states have broken the law … there should be … an honest and open discussion between countries at the international level about whether the current law is good enough, and whether it ought to be changed," he says.

"Whether we should start thinking about the need for an exception to prevent these kinds of atrocious international crimes — genocide or war crimes, crimes against humanity — where civilian lives are seriously at risk."

More than 70 people were killed in the gas attack. ( AP: Aya Fadl )

ABC/wires