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Craig Forcese teaches national security and public international law at the University of Ottawa Faculty of Law. He is co-author of International Law: Doctrine, Practice and Theory and author of a forthcoming book tracing the origins of self-defence and the use of force in international law.

The United States has fired missiles at the Syrian military. This is a dramatic escalation in the horrific Syrian conflict. It is also the most legally doubtful use of military force by a NATO state in recent history.

To be clear, the regime of Bashar al-Assad has itself transgressed most bounds of international law and human morality – most notoriously in what Western states believe was another chemical weapons attack on a civilian population.

But the problem is – as it has always been with Mr. al-Assad's atrocities – that atrocities are not themselves justification for one state to use force against another. This is true whether atrocities are committed with small arms or banned weapons of mass destruction.

Read more: Trump's air strikes in Syria: What we know so far

Boiled to its essence, international law has limited use of force by one state against another to situations of self-defence or circumstances where force is authorized by the UN Security Council.

Here, there is no plausible basis for self-defence. The U.S. has not been attacked by Syria. Nor have its allies. Nor is there any Security Council authorization – thanks largely to the obstinacy of the Russians, allies of the Assad regime.

The result is a legal impasse, one that is not easily circumvented by creative lawyering.

Contrary to popular misunderstandings, there is no separate justification of "humanitarian intervention" or "responsibility to protect" in international law. Those who so claim aspire to a different international law than we have right now. This was most acutely the case in the Kosovo conflict in the late 1990s. There, NATO states engaged in a lengthy air war against Serbia in response to its conduct in Kosovo.

With no plausible basis in self-defence and no Security Council authorization, the best that could be said about that conflict was that it was "illegal but legitimate".

And so we face a repeat scenario: The international community has compelling reasons to punish transgressions of basic and elemental rules, in this case on the use of chemical weapons. Legally, those punishments involve prosecutions for war crimes and crimes against humanity.

But practically, criminal justice solutions require defendants in the custody of police and courts.

That situation does not exist in Syria. And so the U.S. has instead chosen to signal that use of chemical weapon will be met with force. Its defence will likely focus on moral and political legitimacy, whatever the legalities.

But it is worth voicing a note of warning. The stern strictures on use of force were one of the great accomplishments of the post-war world order. It is true that they have been violated in the past. But it is very rare to find an instance where a state did not try (however implausibly) to defend its conduct as self-defence or authorized by the Security Council. Even the states that participated in the infamous 2003 Iraq war proposed a sort of residual Security Council authorization from the original 1990 Gulf War.

We should not dismiss the success of the postwar rules on military force. There are many civil wars. But armed conflicts between states are now rare, to a degree unprecedented in earlier centuries.

Kosovo did mark an exception to this pattern, now joined by the missile strike on Syria. Like in Kosovo, the United States' recent use of force pits that country against a Russian ally. But in other key respects, this is a different situation. Russia has different ambitions than it did in the 1990s – and has forces in Syria it has been prepared to use in support of Assad. Moreover, unlike in the Kosovo conflict, the U.S. acted unilaterally, and not in close concert with its allies (whether they were notified or not).

We may have reason to regret abandoning the straitjacket of international law, regardless of the horrors that motivated this departure. There may be real costs to unravelling well-understood limitations on use of force in this manner, and in the most volatile of possible contexts.

And then there is the long-term consequence. After all, in this period of increased instability, acts done by today's great power – even with the best of intentions – may be mimicked by other aspirants, driven by less palatable objectives.

Be careful, in other words, what sort of international law you wish for.

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