1. There are two main ways a country can legally launch a military operation inside another country: by invitation or via a United Nations Security Council resolution. Neither criteria applies in the current situation; indeed, Russia, an ally of Syria, has a veto as a permanent member of the UNSC.
2. Article 51, the self-defence clause of the United Nations charter. It says a country can take individual or collective action on behalf of a member that is under armed attack. Usually Article 51 would be followed by an international resolution authorizing force. Again, Russia would ensure that doesn't happen.
3. Article 51 in the context of Canada. Defence Minister Jason Kenney suggested that Canada had a right of self defence from extremists who might return to Canada from the civil war in Syria. Legal experts in Washington, who oppose the U.S. bombing campaign, say that justification requires an imminent threat, otherwise it is a preventative war.
4. The Responsibility to Protect doctrine, which has been adopted by more than 150 member countries, including Canada, which helped write it. It says that if a state doesn't live up to its duty to prevent genocide or mass atrocities against its own citizens, it can't invoke sovereignty to keep another country from taking military action inside its borders. Kenney said ISIL was perpetuating genocide in Syria, one that President Bashar Assad can't or won't prevent.
5. The "never again" moral argument. The world said that in relation to the Holocaust. It also said it after the Rwanda genocide of 800,000 people in 1994 and the Bosnian genocide of 1995, which targeted 8,000 Muslim men and boys. The death toll is Syria is already more than 200,000. Paul Heinbecker, Canada's former ambassador to the UN, said earlier this week that there may not be a true legal justification for intervening in Syria, but there is a clear "moral issue."
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