If the United States and its allies responded to Syria's alleged use of chemical weapons by launching a military strike without the approval of the United Nations Security Council, would it be a violation of international law?
Legal scholars are divided on the answer.
For Geoffrey Robertson, a prominent international criminal and human rights lawyer and judge who has argued landmark cases at the European Court of Justice, the European Court of Human Rights, and UN war crimes courts, history is full of precedents that show it wouldn't be.
"The right to intervene, to stop a crime against humanity or deter a crime against humanity, has been with us for many centuries, and it predates the United Nations and it continued after the United Nations," Robertson says.
He says that long before the United Nations was created in 1945, Britain didn't need world approval to attack foreign ports to stop the slave trade, or to lead a coalition to stop Ottoman Empire atrocities and liberate Greece.
And after the UN was created, Ugandan President Idi Amin "killed 75,000 of his citizens and he was overthrown by an entirely legitimate invasion that didn't have Security Council approval. The mass rapes and genocide in Bangladesh [in 1971] was ended by an invasion that didn't have Security Council approval," Robertson adds.
Robertson also cites Kosovo as a relevant precedent, which lawyers at the White House are studying. NATO's 1999 Kosovo bombing campaign halted ethnic cleansing of Albanians by Yugoslav forces but didn't have Security Council authorization. The Independent International Commission on Kosovo later judged the campaign a "legitimate" use of military power.
Robertson says Kosovo is applicable to Syria because both involve crimes against humanity and now, as then, Russia opposes intervention and would use its Security Council veto power to block authorization of force.
That's partly why he says the Security Council is an "unsatisfactory tribunal" to decide urgent moral questions; it can be rendered ineffective by politics and it can be misled -- former U.S. Secretary of State Colin Powell convinced the council in 2003 that Iraq had weapons of mass destruction, an assertion that proved false.
Instead of leaving matters to the council, Robertson would like to see UN Secretary-General Ban Ki-moon appoint a panel of independent international judges to consider the evidence and rule on Syrian President Bashar al-Assad's culpability. Assad has denied ordering the attack.
Beyond stopping crimes against humanity, Robertson also sees legal justification for intervening in the 1925 Geneva Protocol
banning chemical and biological weapons, which Syria signed, and the 1993 Chemical Weapons Convention, which 189 countries, but not Syria, have signed.
"If we're going to have a rule against the use of chemical weapons, there must be consequences for those who do use them," Robertson says. "Otherwise, international law is a chimera, it's not worth the paper it's printed on."
Where Robertson doesn't see a legal rationale is in the UN's doctrine of self-defense, which President Barack Obama has suggested might apply because the use of chemical weapons poses a threat to the United States.
"I think that is really a bridge too far. You can't possibly say that it is an act of self-defense for America to attack Syria," Robertson says. "What you can say is that it's an act of trying to stop an ongoing crime against humanity -- namely mass murder by the Assad government of over 1,000, it seems, of its people, to deter it, by a limited military strike. That, it seems to me, is precisely what America could rightly do without perverting the law in the way that might be suggested by a doctrine of self-defense."
'Illegal But Right'
Representing the other side of the legal argument is Northwestern University law professor and UN expert Ian Hurd, who wrote an op-ed for "The New York Times" headlined, "Bomb Syria, Even If It's Illegal," which he thinks it would be.
In his piece, Hurd notes that the 1925 treaty banning chemical weapons was designed "with international war in mind, not internal conflicts." He says the central legal obligation of all states in the post-1945 world is adherence to the UN Charter, which is unambiguous about when military action can legally be taken against another country.
"On international law, there's pretty clear treaty language that says that countries aren't allowed to use force against other countries, except in self-defense or with the approval of the Security Council," Hurd says. "That's very clear in the UN Charter and so it's been the dominant piece of international law since 1945. It's really the center of the modern international legal order. So there's no exception in that law for humanitarian interventions."
Hurd believes that military intervention in Syria is justifiable on moral grounds, and he would like to see the United States take action for that reason alone.
But the law may be catching up to the moral argument. Hurd says a controversial debate has been going on for some time among scholars over whether UN law "has changed naturally in recent years to make it legal to use force for humanitarian reasons without the council." It's controversial because it relies on the idea that international law can change according to how countries want it to change, depending on how their moral compass compels them to respond to things like genocide.
Hurd says the view that UN law is inadequate to respond to crimes against humanity first surfaced after the 1994 genocide in Rwanda. "The last 20 years or so have really been characterized by the echoes of the Rwandan genocide, where the UN had peacekeepers in Rwanda, and knew that mass killing of innocent people was going on, and it did essentially nothing to stop it on the grounds that international law said it couldn't," he says. "I think we're still dealing with the reverberations of that trauma as it relates to international law, as many people come to feel that's an unacceptable end point for thinking about law."
U.S. Secretary of State John Kerry has called the chemical attack in Syria "a moral obscenity," and French President Francois Hollande has said the only inappropriate response "would be to do nothing."
Hurd says if you accept the growing view that force is justified to prevent large-scale atrocities, "then there is a set of arguments in there for acting in Syria without the council's approval."
On August 29, the British Parliament rejected Prime Minister David Cameron's arguments for a potential military strike. Now the world waits to hear Obama's case.