There is no more doubt. The United States, Great Britain and France will not let Bashar al-Assad’s chemical weapons massacre of Syrian civilians go unpunished. In a few days at most, targeted air strikes could occur to signify to Assad that a red line has been crossed, and there’s no turning back.

We have known for a long time that the Assad regime had chemical weapons. A year ago, President Barack Obama warned that their use would constitute a line that must not be crossed. Numerous sources have led us to believe that they have been used, possibly multiple times, although that can’t be confirmed. But the attack on Ghouta, on the outskirts of Damascus, which last week caused 100 to 150 deaths and many more injuries, changed everything.

Bashar al-Assad’s regime wants to blame this attack on rebel forces. As proof of its professed good faith, it welcomed a mission of experts from the U.N. to investigate; however, it had previously bombed the sites of the chemical attacks to destroy any evidence. On Tuesday, French President Francois Hollande stressed that “everything suggests that the Syrian regime carried out the ‘abject’ act” of a chemical attack. On Monday, U.S. Secretary of State John Kerry had already gone further, arguing that the case for Damascus’s responsibility was “compelling.” The adjectives that he used to describe the massacre as an “obscenity” left no doubt as to President Obama’s desire to react.

The use of chemical weapons in this war constitutes a violation of several international treaties, including the Chemical Weapons Convention, which prohibits their use in any form whatsoever. We are facing a war crime that elicits general disapproval — with the exception of Russia, Assad’s fierce ally. Even Iran, who supports Damascus, has made its voice heard. We know that the U.N. Security Council will never authorize a military intervention on its own because Russia and China will veto it. Nevertheless, the General Assembly should be asked to condemn the Assad regime.

The intervention that the U.S. and its French and British allies are considering is morally and politically justified. By its nature, it will create a coalition of countries that volunteer to commit their resources, which must be as large as possible. Canada must be there. The only questions now should relate to the nature of the intervention. From the outset, it will not be terrestrial so as not to result in a quagmire. It will be aerial, in the form of targeted strikes, similar to what NATO did in Kosovo in the 1990s. It should target airports and armories to weaken the system’s military capabilities.

Such an intervention will change the nature of the Syrian conflict. The United States and other countries have thus far hesitated to arm the rebels, despite promises they have made. The show of force will change that. The hope is that the Assad regime will be forced to engage in negotiations with the rebels, whom it has repressed by any means. It is even possible that the attack on Ghouta was intended to cancel preparations for a conference that was to be held in the next few days in Geneva to initiate talks. To avoid surrender, Assad would like to see conflict spread in the region. This is a danger that Washington cannot ignore. Washington must accurately assess every action that it takes.