THE OBAMA administration faces two distinct challenges in attempting to conclude a diplomatic deal to eliminate Syria’s massive arsenal of chemical weapons. The first is to agree with Russia on a workable plan for declaring, securing and then destroying munitions, chemical precursors and production facilities, with a clear timetable and close international monitoring, and to formalize that plan in a U.N. Security Council resolution. The second is to preserve the U.S. goal of ending the war in Syria through a political settlement that removes the regime of Bashar al-Assad.

So far, the first goal is looking like a huge, if not quite impossible, lift. Secretary of State John F. Kerry said two days of discussions with Russian officials in Geneva were “constructive,” though it wasn’t clear how much progress had been made. The Assad regime, for its part, informed the United Nations that it would join the treaty banning chemical weapons, while making clear that it would embrace the strategy of delay and obfuscation patented by Saddam Hussein and Kim Jong Il. Any declaration of its chemical assets, it suggested, could wait 30 days.

Mr. Kerry rejected that timetable, but the administration has refrained from setting out its deadline for reaching an agreement and has not spelled out its minimum conditions. That could prove to be a mistake if Russia and Syria seek to bog down the talks as a delaying tactic or to resist essential steps. Mr. Kerry would be wise to publicly state minimal U.S. terms and make clear that the United States will use force against Syria if there is no agreement within weeks — say, by the U.N. General Assembly meeting at the end of the month. A congressional authorization of force in the absence of a U.N. disarmament resolution, as has been proposed by bipartisan groups in both houses, would help.

While agreeing with Russia on a plan for Syria’s weapons will be difficult, it looks easy compared with the administration’s aim of advancing a political solution to the war. Though Mr. Kerry and Russian counterpart Sergei Lavrov agreed Friday to work toward convening a peace conference, both Russia and Syria appear intent on using the chemical weapons diplomacy to isolate and undermine the Syrian opposition. Russian President Vladi­mir Putin has said repeatedly that the United States must forswear military intervention in Syria in exchange for an agreement; Mr. Assad said Washington must end its support for the rebels, who only now are beginning to receive U.S. arms deliveries.

Compliance with such demands would make a political settlement impossible and encourage the regime’s current strategy of crushing the opposition by force — a goal that Mr. Putin likely shares. The mainstream Free Syrian Army is already resistant to joining negotiations with the Assad regime; a cutoff of U.S. assistance would eliminate Washington’s leverage. As a practical matter, it would also make any deal on a transitional government impossible: The Assad clique will not yield or be forced from power unless it is losing the war.

Administration officials have been saying that support for the rebels is not up for negotiation and that the option of U.S. force remains open. Any diplomatic solution in Syria will depend on President Obama sticking to those positons. He should not barter Syria’s future for an agreement on its chemical weapons.