In diplomacy, red lines are problematic. While they are needed to signal resolve, they can also invite trouble. “Red lines often become red carpets,” showing that what governments do to challenge their adversaries and test their credibility turns into an indication of how far they can go without inviting retaliation.

In some cases, however, there can be no room for doubt. The prospect of Syrian use of chemical weapons is one such instance. The world must make clear that any such action by the Damascus government will trigger strong multilateral action. Some lines must not be crossed.

In recent weeks, the United States has ratcheted up international attention on the prospect of the use of chemical weapons by Syria. U.S. Defense Secretary Leon Panetta has said intelligence “raises serious concerns that this is being considered.”

Both U.S. President Barack Obama and Secretary of State Hillary Clinton have warned that the use of such weapons would indeed cross a red line and demand a response.

News agencies, no doubt using leaks from official sources, have reported that Syrian military officers have loaded the precursor agents for sarin, the same lethal nerve gas that Aum Shinrikyo unleashed on Tokyo residents in 1995, into bombs that could be dropped from combat aircraft.

Syrian officials deny any such action or intent. Deputy Foreign Minister Faisal Mekdad has said that his government would not use chemical weapons against its own people in the Syrian civial war. “Syria is a responsible country.”

Unfortunately for Mr. Mekdad’s credibility, Syria is one of six countries that has not signed the Chemical Weapons Convention (CWC), which outlaws the use of such weapons.

Experts reckon that Damascus has hundreds of tons of chemical weapons and related materials, including sarin, mustard gas and the nerve agent VX. There are two dangers associated with that arsenal. The first is the obvious threat that the government might use it, even though such a move would clearly violate a powerful norm that prohibits the use of the weapons of mass destruction.

Sadly, this norm — which both reflects and sustains the CWC — has been violated in the past. During the Iran-Iraq war, chemical weapons were used. In 1988, for example, Iraqi despot Saddam Hussein used them on Kurdish rebels: In the village of Halabja, 5,000 people were killed in a single incident.

The second danger is that those weapons might be deployed and then fall into the hands of terrorists by accident or design. Syria’s close ties to organizations like Hezbollah and other armed groups in the region would facilitate the transfer or theft of such weapons in the chaos likely to accompany the end stage of the civil war.

It is possible that the information that the U.S. has received is accurate, but that the reported deployments are intended to signal Damascus’ desperation and to thus strengthen its hand in bargaining over the terms of an eventual handover of power.

Perhaps it is no coincidence that Ms. Clinton was meeting on Dec. 6 with her Russian counterpart, Foreign Minister Sergei Lavrov, on the next moves in the peace process. There have been indications of a shift in the position of Russia, Syria’s strongest supporter, although there is no indication that Moscow is ready to withdraw its backing.

At the same time, rebel groups have made recent gains, although the entire civil war has been a bloody seesaw affair.

Options to deal with this threat are limited. Attacking chemical weapons stockpiles is difficult since their locations are not known and bombing them could scatter the gasses, causing the very casualties that such attacks are intended to avert.

Attempting to grab the weapons is equally problematic, again because it is not known where the weapons are and because it is estimated that some 75,000 troops would be required to find and secure an arsenal of this size. (Reportedly, U.S. forces are deployed in states near Syria to prepare for that eventuality, and to train forces to help out.)

Desperate governments do desperate things. That is why the U.S., along with all other civilized nations, must send one unequivocal message: The use of chemical weapons is a violation of international law and the perpetrators — those who make the decision to use such weapons and those who actually do the deed — will be held to account. Hopefully, this message will not only sober and deter the Damascus leadership and its allies but also send a message to other governments.

Concern over North Korea’s missile program reflects fears about those rockets’ payloads — whether they are satellites or weapons of mass destruction. In addition to its nuclear program, Pyongyang is another of the six countries that have refused to sign the CWC.

For all of North Korea’s provocations, the government in Pyongyang has demonstrated a shrewd understanding of the limits of belligerence. Yet there is real concern that, in a crisis, the North’s leadership might lash out with weapons of mass destruction as a way of bargaining or to inflict as much pain as possible before it disappeared.

So, the message to the Syrian leadership about the inviolability of the chemical red line is intended for Pyongyang, and other governments in similar situations, as well.