You might be wondering why President Obama looks likely to strike Syria over the deaths of several hundred civilians from nerve gas — when the Assad regime’s slaughter of tens of thousands of Syrians by conventional weapons has triggered no such response.

You are right to wonder, because the answer to that question will determine whether a U.S. military strike does more harm than good.

There is one obvious reason for military action. The use of chemical weapons is a huge breach of a longstanding global taboo. It is especially horrific to watch videos of young children foaming at the mouth or gasping for breath, with their little bodies piling up like cordwood, in the Damascus suburb that was attacked last week.

The use of chemical weapons on a massive scale flouts a historic taboo in place since the widespread use of gas on World War I battlefields, and not broken since the 1980s, when Saddam Hussein gassed Kurds and Iranians during the Iran-Iraq War.

One hundred fifty nations (not including Syria) signed a Chemical Weapons Convention in 1993 banning their use. A failure to respond to this breach could erode international norms against weapons of mass destruction — norms that President Obama takes very seriously.

And there appears little doubt that it was the regime — not the rebels — that mounted the gas attack. Unlike the rebels, the forces of Bashar al-Assad have both the weapons and the delivery vehicles. They have been conducting smaller chemical attacks over the last year, without any Western military reaction, so they probably assumed they could use such weapons again to clear civilians out of a hotly contested area.

Indeed, U.S. intelligence officials reportedly have intercepts of panicked phone calls between Syrian defense officials and leaders of a government chemical-weapons unit over the attack (which may have claimed more victims than intended).

Yet the breaking of the taboo on chemical-weapons use doesn’t seem like an adequate explanation for a possible military attack.

This president is clearly reluctant to get militarily involved in the Syrian conflict. He rejected advice from his most senior military and civilian officials, a year ago, to arm secular rebel commanders who had been vetted by the CIA. As a result, these moderate commanders, short of serious weapons and often lacking bullets, have been marginalized by hard-line jihadi militias that get plentiful money and weapons from the Arab gulf.

As a result, a new al-Qaida heartland has been established in eastern Syria, spilling over into western Iraq, attracting Arab jihadis who could threaten the West in the future. Floods of Syrian refugees are destabilizing Lebanon and Jordan. This hasn’t moved Obama to take military action (even promised small arms haven’t been delivered to vetted rebel leaders).

So what is driving Obama now?

My answer: The latest Syrian chemical attack boxed Obama in. One year ago, he warned that the Syrian regime’s use of chemical weapons would cross a “red line.” He repeated that warning over and over, yet failed to respond to previous chemical attacks.

This latest, massive Syrian chemical attack appears to have embarrassed him into action. Unless Obama makes good on his warning, his credibility will be in tatters — and his “red line” over the building of an Iranian nuclear weapon will look like a joke.

So the president probably feels he has been cornered into authorizing some form of a military strike. Administration officials have repeatedly leaked the message that any action will be short, minimal, and meant only as a warning to deter future Syrian chemical attacks.

They insist that any strike won’t aim to change the military stalemate between regime and rebels. They say Assad’s exit can only be achieved via political negotiations.

Sadly, this reluctant intervention — divorced from a broader strategy — may make a negotiated peace even less likely. “This sort of hesitant, half-hearted action may be worse than no action at all,” says Faysal Itani, a Syria expert at the Atlantic Council’s Rafik Hariri Center for the Middle East. I agree.

That’s because a limited strike, say, on the headquarters of units that conducted the chemical attacks, will achieve little. Assad may refrain from using chemicals again, but he will continue to slaughter civilians with bombs and artillery. He will brag that he surmounted America’s efforts to unseat him. He will be less, not more, likely to negotiate — or to contemplate resignation.

To succeed, any U.S. attack must be tied to broader policy objectives. Having backed into military action, Obama should use this excuse to degrade Syria’s war-fighting capability — at least destroying some regime aircraft. (He should also ensure that moderate rebel commanders get serious arms so they can organize a fighting force that can counter the jihadis — and can morph into a new national army if Assad goes.)

Only when the military stalemate is broken will Assad — and his Russian and Iranian backers — be forced to consider serious talks with the rebels. Otherwise — even without the horrors of sarin gas — tens of thousands more Syrians will surely die.