Oona A. Hathaway and Scott J. Shapiro are professors at Yale Law School.

As sickening images of Syrian children apparently gassed by their own government fill U.S. newspapers, a deeply understandable desire is growing to make Syrian President Bashar al-Assad pay for his flagrant violations of law and basic human decency. But unleashing even limited military force without U.N. Security Council authorization would threaten the fundamental principles of the international legal system and, in so doing, put us all at risk.

It is a bedrock principle of international law that one state may not unilaterally attack another except in self-defense. That is true even when a state openly violates laws such as the 1925 Geneva protocol prohibiting the use of chemical and biological weapons. In the absence of Security Council authorization, international law prohibits the use of military force to enforce international law. No state may play police officer to the world on its own say-so. Secretary of State John F. Kerry knows this; at his confirmation hearings in January, he said, “I think a U.N. resolution is a necessary ingredient to provide the legal basis for military action in an emergency.”

It wasn’t always this way. For centuries, war was the means by which violations of the law were enforced. Every sovereign state could act as the world’s police officer, and the system was a disaster: No one had the power to say when the rules had been broken or rights had been violated, and therefore everyone had the power to go to war.

Led by the United States, the world rejected this system, first in the Kellogg-Briand Pact of 1928 and again in theUnited Nations Charter in 1945. Nations agreed that, absent collective agreement among the members of the Security Council that rules violations were ongoing and must cease, no state had the right to unilaterally decide otherwise.

Those advocating intervention in Syria point to the 78-day U.S. air attack in Kosovo in 1999 as evidence that force can be used when humanitarian disaster looms. At the time, many experts defended the attack as “illegal but legitimate.” But in the world of international law, there is no such thing. When a nation as powerful as the United States repeatedly breaks the law, that law ceases to be law.

If the United States is able to decide for itself — or with the help of friendly states — when international law may be enforced with force, everyone else may do the same. When Russia intervened in Georgia in 2008, it cited humanitarian concerns and the “responsibility to protect” peacekeepers and Russian citizens. The international community quickly rejected these arguments, as well as claims that Russia was acting in self-defense. But if the United States may intervene in Syria without Security Council authorization for humanitarian purposes, why couldn’t Russia intervene in Georgia?

The issue is the Security Council’s failure to act. China and Russia have stood in the way of an effective response to the atrocities in Syria, but with Britain’s resolution to the council, they now have an opportunity to correct course. If they fail to do so, the international community ought to direct its ire at these protectors of the Assad regime who enable the use of poisonous gas on innocent children.

If these nations are going to be gatekeepers and if the system is to continue functioning, they are going to have to play a role in it. That means that, when there is an open and flagrant violation of the most basic norms of the international legal system, they cannot stand in the way of a response. If they do, they should be held publicly responsible for the unfolding horrors.

The United States could begin by cutting off all contracts to purchase military supplies from Rosoboronexport, the state-owned Russian arms dealer that has provided many of the weapons behind Assad’s atrocities and from whom the United States has purchased weapons to supply Afghan forces.

Military force is not guaranteed to work, nor is it the United States’ only option in Syria. Alternatives include broader economic sanctions to disrupt the supply chain on which the Syrian government relies to carry out its attacks on civilians; freezing the assets of Syrian government officials and relevant officials around the world; and stepping up support for opposition forces. These measures would require time to build international support, but they might prove more effective in the long term.

The United States is on the precipice. If it uses force on its own or with a cobbled-together “coalition of the willing,” it will undermine the prohibition on war that has been the bedrock of the post-World War II era. Assad’s actions in Syria are terrible, but breaking international law by intervening without Security Council approval would send us all down a road that could lead to even worse.