The legal case for the U.S. to strike against Syria in retaliation for its alleged use of chemical weapons against its citizens is straightforward to some international law observers.
Simply put, any attack without UN consent would break international law.
"There are a couple of important points that intervention needs to work around. First, that the United Nation's charter forbids countries from using or threatening force against other countries," said Ian Hurd, an associate professor of political science at Northwestern University in Illinois. "That’s a pretty fundamental piece of international law.
"And it's pretty clear in the UN charter ... that you can only use force in self-defence or with [UN] Security Council approval."
And in the case of Syria, Hurd said, "the U.S. using force against Syria without [approval of] the council is very clearly illegal."
Article 2 (4) of the UN charter states that "all members shall refrain in their international relations from the threat or use of force against the territorial integrity or political independence of any state." But the UN does allow for states to use force for self-defence and, under Chapter 7, authorizes the Security Council to take action "to maintain or restore international peace and security."
It adds that nothing in the charter "shall impair the inherent right of individual or collective self-defence if an armed attack occurs against a member of the United Nations."
In an op-ed piece for the Washington Post, Yale Law School professors Oona Hathaway and Scott Shapiro argue that the desire to make Syrian President Bashar al-Assad pay for his violation of law for allegedly gassing and killing more than 1,000 people is understandable.
Would threaten fundamental principles of international law
"Unleashing even limited military force without UN Security Council authorization would threaten the fundamental principles of the international legal system and, in so doing, put us all at risk," they write.
Even though Syria may have violated the 1925 Geneva protocol by using biological and chemical weapons, "it is a bedrock principle of international law that one state may not unilaterally attack another" except in self-defence or with Security Council authorization, Hathaway and Shapiro wrote.
"No state may play police officer to the world on its own say-so," they write.
The Obama administration is reportedly working on preparing a legal case for intervention. Before the British parliament shot down Prime Minister David Cameron's attempt to get approval for military action, his office had prepared a legal report to justify intervention.
The report claimed that even without UN Security Council approval "the U.K. would still be permitted, under the doctrine of humanitarian intervention, to take exceptional measures including targeted military intervention in order to alleviate the overwhelming humanitarian suffering in Syria."
Responsibility to protect
There has been movement in recent years to justify use of force under the doctrine of "responsibility to protect" (also known as R2P), adopted by UN member states in 2005. The idea has been championed by a number of organizations, including Canadians like former foreign affairs minister Lloyd Axworthy, former attorney general Allan Rock and former Liberal leader Michael Ignatieff.
As Rock and Axworthy recently wrote in a piece for the Globe and Mail, R2P "established the basis for military action, as a last resort, to protect civilian populations from mass murder" and that "R2P can and should be used as the basis for action in Syria."
They believe that the NATO bombing of Kosovo in 1999 to stop Serbian aggression was an appropriate precedent and suggest that R2P trumps the UN charter and any inaction by the Security Council.
"Although the 2005 agreement contemplated a Security Council resolution authorizing military intervention, member states surely did not intend that urgent humanitarian responses would be hostage to vetoes unreasonably exercised out of self-interest by one or more of the permanent five council members," they wrote.
But Hathaway said that's just not so, and that the doctrine does not allow intervention outside of the UN charter. In an email to CBC News, she referred to UN Secretary-General Ban Ki-moon, who in 2009 said that "the responsibility to protect does not alter, indeed it reinforces, the legal obligations of member states to refrain from the use of force except in conformity with the charter."
"So, yes, if a state attacks another state without Security Council approval but cites 'responsibility to protect,' they would still be breaking international law," Hathaway said.
Ignatieff agreed with Hathaway's legal interpretation of the doctrine but said action should still be taken.
"If the Security Council fails to approve military action against Syria, the action would be illegal, in strict terms, under current international law," Ignatieff, a professor at the Munk School of Global Affairs in Toronto, wrote in an email to CBC News.
But according to the concept of "responsibility to protect," the action could be justified morally, Ignatieff said, on the grounds that all states have a common interest in protecting civilians against the use of such weapons.
"A moral case, according to responsibility to protect doctrine, must include a commitment to confine the operation strictly to deterring future use of chemical weapons and to avoid civilian casualties or harms," he said. "In other words, a moral case faces a high standard of justification in the absence of a legal authorization from the Security Council, but it is a case that can — and in my view — should be made in this case."
With files from Sean Davidson and The Associated Press
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