If the United States becomes convinced that Bashar al-Assad's regime has started using chemical weapons against its own citizens, the "red line" set down by President Barack Obama will have been crossed. If the U.S. initiates military action as a consequence, it will provoke the same questions of legality that arose during NATO's bombing of Kosovo 14 years ago.
In early 1999, after Serbian police and military forces had massacred civilians, NATO began a bombing campaign in what Michael Ignatieff called a "virtual war" against Slobodan Milosevic's forces. After 10,000 strikes against key military targets, the Serbs conceded to NATO's demands on June 10. Canadian fighter jets were major participants in the aerial bombing. There were military and civilian casualties on the Serb side along with the accidental bombing of the Chinese embassy. NATO did not suffer a single casualty.
The NATO military intervention was without the consent of the United Nations Security Council after it became clear that Russia would veto any proposal to intervene in the former Yugoslavia.
Leaders of NATO countries knew that under the UN Charter, there is a prohibition against the use of force against any state by another state or alliance. It is regarded as one of the most important principles of sovereign equality and territorial independence under the security framework established in the UN Charter after the Second World War. The right of self-defence under Article 51 is strictly limited to a prior armed attack upon one sovereign state by another.
The military intervention by NATO in Kosovo was clearly illegal under the UN Charter and international law. However, there were international experts and jurists, including this author, who regarded the intervention as illegal but legitimate. It has been called the Kosovo exception: It creates, de facto, an illegal but legitimate military intervention.
Critics warned that the exception could be used for illegal and illegitimate military interventions. They were proved correct with the illegal intervention led by the United States in Iraq. However, those who supported the Kosovo intervention insisted that in very limited circumstances, when the UN Security Council fails to stop civilian atrocities, there is a moral imperative to act.
NATO itself did not offer an international-law justification for its Kosovo intervention, but the need to provide frameworks for future interventions eventually gave rise to the world leaders endorsing the Responsibility to Protect principles at the UN World Summit in 2005. That development is now in jeopardy after the very unfavourable reaction by Russia and China to the 2011 intervention in Libya. Those countries have now refused to permit the Security Council to pass any resolution to stop the massacre of civilians in Syria – despite its death toll having surpassed 70,000 men, women and children.
So, if the United States alone, or with NATO, did intervene in Syria in response to a proven use of chemical weapons, it would again be without Security Council authorisation. We would likely witness the creation of a Syria exception on the same basis as the exception used for the Kosovo intervention.
Again, there would be claims that the intervention is a violation of Article 2(4) of the UN Charter, and reminders that the use of self-defence under Article 51 of the Charter is not applicable.
However, some may argue that the use of chemical weapons against civilians in Syria could result in new reasons for legitimate self-defence intervention. This argument would focus on the danger to Turkey, a NATO member, as well as to Jordan and even Israel if the chemical weapons could drift across borders or get into the wrong hands, such as those of Hezbollah.
There is little hard international-law justification for such support for an intervention; certainly not if the intervention were aimed at ousting the Assad regime. The United States and its allies, including Canada, should instead rely on the older argument: When a regime sees no limits to what it may inflict against its own citizens, including the use of weapons of mass destruction, then some types of intervention may be technically illegal under international law, but the arc of history will determine that they are legitimate.
Errol Mendes is a professor of constitutional and international law at the University of Ottawa. He was also an advisor on the Independent Commission on Kosovo headed by Justice Richard Goldstone.