Last week, Syrian President Bashar al-Assad demonstrated his utter disdain for U.S. efforts to launch Syrian peace talks next month in Geneva.

Even as Secretary of State John Kerry was roaming the region seeking Arab backing for talks, the Syrian regime reportedly launched a chemical weapon against civilians in a northeastern Damascus suburb. The fourth such episode, this one sent at least 50 victims to the hospital with bronchial and muscle spasms.

Assad once more thumbed his nose at President Obama's "red lines" with impunity, while insisting he will run in "elections" in 2014. Could the message possibly be clearer? Armed by Moscow and Tehran, buoyed by Iranian and Hezbollah fighters helping him regain lost ground, Assad thinks he is winning.

No wonder. The White House still refuses to do what is necessary to force him to take talks seriously, thus ensuring that any Geneva meeting will fail.

The administration is correct to argue that a negotiated peace would be the best way to stop the Syrian carnage. It would also be the best way to stop the fighting from spilling over further into Turkey, Lebanon, Iraq, or even Israel, and to prevent a new al-Qaeda base in the heart of the Middle East.

Yet, at present, Washington has little or no leverage with which to squeeze Assad into accepting the principles underlying the Geneva talks: that both sides would agree to a transitional government (minus Assad), which would take control of the country and prepare for elections.

Assad rejects that premise. Moscow (the cosponsor of the talks, on whose good offices Kerry is relying) backs Assad. The dictator's stonewalling "reflects his belief that the Iranians and Russians will pull his chestnuts out of the fire," says Fred Hof, a former special adviser on Syria at the State Department.

So, even if Syrian opposition leaders agree to attend talks, what's the point?

True, Kerry warned last week that, if Assad won't negotiate in good faith, the United States might increase its support for the opposition. But U.S. officials still insist that any new support will only be "nonlethal," meaning no weapons. This has convinced Assad he can ignore U.S. threats.

The dictator knows it took two months for Washington to deliver promised food rations and medical kits to Syrian opposition militias that were CIA-vetted to ensure they are not linked to jihadis. The administration still has not delivered communications equipment promised in April.

Meantime, non-jihadi rebels whom Washington supposedly backs are losing ground to Assad's forces because the rebels don't have enough bullets. (Jihadi fighters, on the other hand, seem to have no trouble getting guns and cash from rich Gulf Arabs.) No wonder Assad feels free to take a hard line on Geneva talks.

The talks might have a chance, however, if Obama were willing to play hardball with Assad. The dictator and his allies ignore rhetoric, but respect strength.

Playing hardball would require two essential steps.

First, Assad would have to be convinced he could lose the military battle. The administration would have to listen to Gen. Salim Idriss, commander of the rebels' Supreme Military Council (SMC), who wrote to Kerry: "For the negotiations to be of any substance, we must reach a strategic military balance, without which the regime will feel empowered to dictate, or at least stall for precious time to achieve gains on the ground under the cover of diplomacy, while fully sustained logistically and militarily by Russia and Iran."

To achieve a strategic balance, Idriss wrote, the United States should provide rebel forces under the command of the SMC with sufficient advanced weapons "to sustain defensive military capabilities in the face of the Assad forces."

In other words, for diplomacy to work, vetted rebel groups first need more, and better, arms.

Which brings us to the second step. Those weapons don't necessarily have to come from the United States, although failing to provide them severely undercuts U.S. leverage with its enemies and its allies. The weapons can come from Gulf states - or from Britain and France if the European Union drops its arms embargo on Syria. But Washington must play the lead role in ensuring that the aid is coordinated in a way that strengthens responsible rebel groups.

The administration claims to support the SMC and Idriss, but does little to prove it. It has outsourced the job of arming the rebels to Qatar and Saudi Arabia, but failed to persuade them to coordinate their aid through the military council. Instead, they arm their favorite militias (often salafi militants). This undercuts Idriss' efforts to create a coherent rebel army that could marginalize jihadi groups.

The Qataris and Saudis have felt free to ignore Washington because, despite Kerry's efforts, they see no sign the White House is fully invested on the Syria issue. If these talks are to have any chance of success, that perception must change.

Obama must unite the Gulf states, Turkey, and involved European nations around a coherent policy for arming the rebels. He must persuade the Russians and Iranians that, so long as their arms flow to Assad, he will ensure that vetted rebel groups get equal weapons.

Such tough tactics are required to convince Assad that there is no alternative but to make way for a transitional government via diplomacy. Otherwise, the Geneva talks are doomed before they start.