In a previous editorial, the Law Tribune Editorial Board expounded upon the importance of remaining vigilant in upholding the rule of law when determining the response to Syrian President Bashar al-Assad's criminal use of chemical weapons against his own people. We expressed relief that President Barack Obama did not act unilaterally, but instead sought congressional authorization for potential armed intervention, even while we took no position on what action, if any, should be sanctioned.

On Monday, just as the Obama Administration and Congress were engaging in a standoff over whether to authorize the use of military force against Assad, a possible diplomatic solution opened up. In a deal proposed by the Russians, Assad orally agreed to sign on to the Chemical Weapons Convention, to place his chemical weapons cache under international control, and halt further production. Although it is not known whether this first step will mature into an enforceable agreement, it was widely seen as giving all parties some breathing room. In the meantime, Congress will continue to debate whether to authorize a strike against Assad's forces, to keep up the diplomatic pressure.

The American people are not in favor of U.S. military involvement in Syria according to recent polls; many members of Congress are also opposed or on the fence. Others are worried about "saving face" for the president and the country, for having earlier warned that the use of chemical weapons would be a "red line" that must not be crossed. But what is lost in the emotion of the debate is the fact that under international law congressional approval alone is not a sufficient basis for unilateral military action against Syria. There is quite a difference between congressional authorization to commit the country to war in these circumstances, and the legality of doing so.

The United Nations charter prohibits signatory countries from waging war except for purposes of self-defense against an armed attack, or unless approved by the U.N. Security Council. Punishing wrongdoing is not viewed as a justification for use of military force under international law. Nor is the use of force specifically permitted on purely humanitarian grounds, even in response to crimes as egregious as the gassing of 1,400 Syrian civilians in their beds.

There is a strong argument to be made that initiating even limited military force without Security Council approval would jeopardize the fundamental principles of the international legal system, putting all of us at risk. In an opinion piece recently published in the Washington Post, Yale professors Oona Hathaway and Scott Shapiro wrote that "[w]hen 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."

The authors advocate broader economic sanctions to disrupt the supply chain, freezing the assets of government officials, and stepping up support for opposition forces.

Hans Blix, former head of the U.N. Monitoring, Verification, and Inspection Commission, warns that unilateral U.S. military intervention would "inspire other countries to do likewise and thus contribute to international instability." He proposes putting pressure on the third party countries that are supplying the factions in Syria to demand that their clients accept a cease fire.

Still, one can argue that policy decisions should be driven by ethical, as well as by legal and political principles. Some crimes – genocide, slavery, and piracy – are viewed under international law as inherently unlawful and are prohibited even in the absence of a governing treaty. The use of chemical warfare is not in this category, although it probably should be.

Syria signed the Geneva Protocol for the Prohibition of the Use in War of Asphyxiating, Poisonous or Other Gases, and of Bacteriological Methods of Warfare in 1968. However, Syria is not party to either the Biological Weapons Convention of 1972, nor the Chemical Weapons Convention of 1993. Regardless of whether Syria is or becomes a party, however, such treaties rely on the Security Council for enforcement. Russia and China, so far, have said they would not agree to authorize a strike against Assad and originally denied that he even used gas on his people.

Rule 74 of the International Committee of the Red Cross's statement of customary international humanitarian law provides that the use of chemical weapons is prohibited, and that "state practice establishes this rule as a norm of customary international law applicable in both international and non-international armed conflicts." It is important to note that the Obama Administration has argued that the prohibition on the use of chemical weapons is an "international norm" that could not be violated without consequences. Administration spokesmen have avoided saying that it is an "international law," because it is not the law.

Under the theory of jus ad bellum, or the theory of just war, there are moral reasons for disregarding the law. It has been reported that the Obama Administration is looking to the 1999 U.S.-led NATO air attack on Kosovo as a precedent for taking military action in Syria absent Security Council approval. The bombing of Kosovo was justified under the humanitarian principle that there exists a "responsibility to protect," and it was later termed an "illegal but legitimate" use of force by the Independent International Commission on Kosovo.

However, the "responsibility to protect" doctrine, including its use in Kosovo, has been widely objected to by other states, which prevents it from being considered as customary international law. The International Court of Justice Statute defines customary international law in Article 38(1)(b) as "evidence of a general practice accepted as law." Humanitarian intervention has been rejected by other states in cases where far more lives were lost than in Syria. India's claim of humanitarian intervention in the civil war between West and East Pakistan (Bangladesh) was not endorsed by a single country. Vietnam's military intervention into Cambodia that ended Pol Pot's murderous reign was rejected by a majority of the U.N. General Assembly, including a negative vote by India. In 1987, in the case of Military and Paramilitary Activities in and Against Nicaragua, the International Court of Justice concluded that binding customary law prohibits coercive military intervention into the internal affairs of another state.

When considering a resolution authorizing a strike against Assad in the event diplomacy fails, Congress will have to decide whether to follow established international law or to conclude that "illegal but legitimate" use of force is preferable over not taking any action. Attempts will be made to build the case that air strikes against Syria are legitimately part of our self defense, even though we did not experience an armed attack.

Or it might be argued that the norm prohibiting chemical warfare has morphed into customary international law, and that military force therefore is justifiable without Security Council approval. Conceivably, a resolution will be offered in Congress that requires Security Council approval before any strike is authorized.

The Obama Administration and Congress can and will argue about whether independent military intervention in Syria is justified, but it is not obvious that they can argue that it is legal.