Well, Bashar, now you’ve done it. We’ve seen that killing tens of thousands of your fellow Syrians with conventional weapons is more or less acceptable to the civilized world, as evidenced by the fact that the civilized world hasn’t stopped you from killing tens of thousands of your fellow Syrians with conventional weapons.

But now, if U.S. Secretary of Defense Chuck Hagel is to be believed -- and Hagel isn’t a man looking for excuses to get entangled in the Middle East -- you have apparently used chemical weapons (the nerve agent sarin, to be specific) on civilians.

If you recall, President Barack Obama drew a “red line” for you: no use of chemical weapons in your brutal attempt to put down the uprising against your regime. Any use of such weapons (even any “moving around” of such weapons) would “change my calculus,” Obama said, “change my equation.” In other words, welcome to the day in which the calculus might just be changing.

Hagel, speaking to reporters in Abu Dhabi, said that U.S. intelligence has come to believe -- like the Israelis, the French and the British before them -- that President Bashar al-Assad’s regime seems to have used sarin “on a small scale.”

I spoke with Representative Mike Rogers, the Republican chairman of the House Intelligence Committee, who said that he thinks the Obama administration is hesitant to face the truth that the Assad regime has already used these sorts of weapons. “Clearly the administration doesn’t want to see this,” he said. “We have lost the confidence of the Arab League and the Syrian opposition because of our inaction.” Rogers said he was convinced at least a month ago that Syria had used a small quantity of chemical weapons against civilians.

Before we get to the meaning and potential consequences of this horrifying news, a brief primer on sarin, which was invented in Nazi-era Germany for use as a pesticide, and which was most famously used in the Tokyo subway attack by the Aum Shinrikyo cult in 1995 and against Kurdish Iraqis during Saddam Hussein’s genocide campaign.

Convulsions, Death

Exposure to sarin gas, even in small doses, will cause victims to experience vomiting, diarrhea, an increased heart rate, eye pain and drooling. In larger amounts, the exposure can cause convulsions, paralysis, breathing difficulty and, within a short time, death. Sarin stops the body from controlling muscle and gland functions, according to the Centers for Disease Control and Prevention, and this causes muscles to twitch and breathing to become faster and irregular. Eventually, the body is overwhelmed, and breathing stops. On a single day in 1988, in a single town, Saddam’s forces killed some 5,000 Iraqi Kurds using a combination of mustard gas, sarin, VX and tabun.

Assad has committed many terrible crimes against his people, but if these latest reports are confirmed, he will have entered into the pantheon of the modern era’s worst war criminals, just as Saddam did in 1988. Back then, Saddam was considered an ally by the U.S. (he was in the midst of a war with Iran). So, to the everlasting shame of President Ronald Reagan, the U.S. did nothing to stop his genocide.

Here is a chance to make amends, not to the Iraqi victims of chemical weapons, but to the notion, shared by all non-sociopathic people, that there are things that human beings simply don’t do to each other. Obama has been rightly criticized for drawing a line in the sand on chemical weapons. His statements communicated to Assad that he was free to massacre his people using conventional means without fear of undue American molestation. If Assad has indeed crossed this line, he has invited armed intervention. And if that intervention doesn’t come, the consequences -- moral and political -- could be long-lasting.

So what should Obama do? Sitting, as he is today, with President George W. Bush at the dedication of Bush’s new library, Obama is certainly aware of the risks of taking action based on faulty, incomplete or flat-out wrong intelligence. So nothing at all should be done until the evidence is absolutely clear that sarin was used deliberately by the Assad regime.

Making Choices

At which point Obama has some choices to make. He could use force to begin destroying or safeguarding Syria’s chemical weapons stockpiles. This is dangerous, of course, and could even involve the insertion of U.S. special operations forces trained in such work. Obama’s other options include establishing no-fly zones and openly arming Syria’s rebels, steps that could lead to the quicker collapse of the regime. No-fly zones are expensive and hazardous to maintain, but they would go a long way toward degrading Assad’s power. Arming the rebels is similarly fraught, because many of them are the sorts of people who seem likely to one day turn these weapons back on the U.S. or its friends.

There are no good choices -- good outcomes in Syria are impossible to imagine. But if it is proved to a certainty that Assad is trying to kill his people with chemical weapons, then Obama may have no choice but to act, not only because he has put the country’s credibility on the line (Iran and North Korea are undoubtedly watching closely), but also because the alternative -- allowing human beings to be murdered by a monstrous regime using the world’s most devilish weapons, when he has the power to stop it -- is not a moral option for a moral man.

In a discussion about the long-term consequences of interventionism I had with him when he was still a senator, Obama said, “What I don’t want to see happen is for Iraq to become an excuse for us to ignore misery or human-rights violations or genocide.”

In Iraq, part of the challenge for U.S. intelligence was that Saddam was suspected of hiding non-conventional weapons. It is easier for intelligence services to find proof of chemical weapons in active use than chemical weapons that are hidden.

Rogers thinks it is time for action. “We have unique capabilities to make it very difficult if not impossible” for the Syrian regime to launch further chemical attacks, he said. “And I’m not talking about boots on the ground, period.”

If U.S. intelligence doesn’t find definitive proof that these weapons are being deployed, Obama should not act. And if he has to act, he should not commit U.S. ground troops to hold Syrian territory. (As the former secretary of defense, Robert Gates, has said, “Any future defense secretary who advises the president to again send a big American land army into Asia or into the Middle East or Africa should have his head examined.”)

But if Assad is guilty of using chemical weapons on his own people, we have reached the moment at which force is required to match Obama’s words.

(Jeffrey Goldberg is a Bloomberg View columnist. The opinions expressed are his own.)

To contact the author of this column: Jeffrey Goldberg at goldberg.atlantic@gmail.com.

To contact the editor responsible for this column: Timothy Lavin at tlavin1@bloomberg.net.