By Jennifer Leigh and Jennifer Leaning
The United States can hesitate no longer to enforce international humanitarian law in the Syrian war. The clear-cut use of chemical weapons against civilian populations in northern Syria on Tuesday April 4th violates long-established legal doctrine against the use of these indiscriminate and brutal weapons in war and has inflicted death on scores of civilian women and children. The Syrian government attack in Khan Sheikhun on Tuesday demonstrates that without decisive and definite action against crossing the red line laid out by the Chemical Weapons Convention, the cycle of grave breaches of international law in this drawn-out conflict is bound to continue.
As public health and human rights analysts of the effect of war on Syrian refugees and internally displaced people, we know that Tuesday’s attack signals the collapse of the last refuge for civilians in Syria. Nearly 7 million Syrians are internally displaced, having fled their homes to escape fighting or having been forcibly evacuated by conflict zones in the regime’s green buses. Many of these civilians are now packed in makeshift housing around the city of Idlib, extending into areas controlled by the Free Syrian Army in parts of Aleppo, Hama, and Latakia governorates. This area, whose pre-war population was 2.5 to 3 million, has now ballooned to over 4 million. It is bursting with civilians who have come from Aleppo, rural Damascus, Hama, and other parts of Syria now controlled by the government or held by ISIS.
The people here are essentially trapped between the Turkish border and territory controlled by the Syrian government. This area houses the only population that has been allowed to move to find safety, and now there is nowhere left to go. The one open crossing with Turkey at Bab al Hawa is restricted, allowing passage for only humanitarian aid, medical personnel, and patients. A few people may find passage through the Turkish protected area of northern Syria, between the two Kurdish strongholds, but this route is perilous and uncertain, and requires money, papers, and connections.
The result is an effective cul-de-sac enclosing millions of people surrounded by government forces. Tuesday’s attack in this area demonstrates that none of these millions are safe from President Assad’s demonstrated willingness to embark on unlawful attack on civilians with no chance of flight, deploying unlawful weapons of war. There is a high urgency for the international community, particularly the United States and Europe, to take action to protect these civilians.
The use of chemical weapons is a grave violation of the Chemical Weapons Convention, one of the most crucial linchpins in Hague Law defining bans on use of certain weapons in armed conflict. This Convention entered into force in 1997, has 192 state parties, including Syria and Russia, as well as the United States. Prohibitions against the use of chemical weapons date back to the Geneva Protocol of 1929, also ratified by Syria, reinforced in 1972 and 1993. The use of chemical weapons also falls within the list of crimes under the jurisdiction of the International Criminal Court as established by the Rome Statute.
Now in its seventh year, the conflict in Syria has confronted the international community with evidently insuperable challenges. One reason for the duration and intensity of this war has been the diplomatic impasses and failures to which the US government has contributed. President Trump has backed Wednesday’s forceful words with force, which may shift some of the dynamic in Syria, However, what is most needed is not US unilateral action, but a deliberate effort to break the international logjam by coordinating and leading the strongest possible political and security joint action to punish tactics that the world knows, and President Trump has noted, have repeatedly crossed “many, many lines.”
Jennifer Leigh is a senior researcher at Harvard FXB.
Jennifer Leaning is Francois Xavier Bagnoud Professor of the Practice of Health and Human Rights at the Harvard T.H. Chan School of Public Health.
 In September 2013 Syria informed the UN Secretary General of the State’s accession to the Chemical Weapons Convention.