After World War I, the use of chemical weapons was prohibited by international treaties. The use of chemical weapons against civilians is now recognized as a war crime and a crime against humanity. The importance of the prohibition is so great that in 2013 President Barack Obama threatened to respond to Syria’s use of chemical weapons with force.
But Russia has blocked efforts by the U.N. Security Council to investigate Syria’s use of chemical weapons, to refer perpetrators to the International Criminal Court and to authorize countries to use force to prevent future chemical weapon attacks. Many observers believe this is payback for Syrian leader Bashar al-Assad allowing Russia to maintain the immense naval base of Tartus in Syria.
Then, in April 2018, the Trump administration, joined by the U.K. and France, fired 103 missiles at three Syrian chemical weapons production and storage facilities. That assault was launched after reports from Syria that more than 40 people were killed in a chemical weapons attack in rebel-held Douma.
The purpose of the airstrikes was to halt Syria’s continuing use of these deadly weapons. For a while, they did the trick.
Now, Syria is preparing for a major offensive in Idlib. The Trump administration warned in September 2018 that if chemical weapons are used again by the Syrian government, the U.S. would deliver a counterattack much more severe than the April 2018 airstrikes.
The conventional view is that use of force against another state is lawful only with U.N. Security Council approval, or in self-defense in response to an armed attack. Neither of these justifications is applicable to the present situation. Russia is preventing the Security Council from acting. And the U.S. isn’t under attack – Syria is using chemical weapons against its own people.
So were the U.S., French and U.K. strikes in April a violation of international law? Would a new attack now violate that law?
New standard: Humanitarian intervention
Having international law on the U.S. side can help enlist allies to the cause being championed by the U.S. In the case of Syria, it can result in greater pressure on Syria to forego chemical weapons in the future.
At an emergency session of the U.N. Security Council the day of the April 2018 airstrikes, the U.K. posed a third justification that would allow one country to use force against another. That exception: humanitarian intervention to prevent the use of chemical weapons against civilians.
At the same session, the U.S ambassador told the Security Council that the U.S acted in “lock step” and “in complete agreement” with the U.K., thereby adopting its legal rationale.
This was the first time in history that the U.S. and U.K. used the legal argument of humanitarian intervention to justify a use of force.
In the past, the two countries have said that actions to halt ethnic cleansing of Kosovar Albanians in Kosovo in 1999 and to save the Yazidis on Mount Sinjar in Iraq in 2014 were morally necessary but they declined to rely on a legal argument. They preferred to say the situations were sui generis, or without precedent.