Moammar Gadhafi faces a stark choice: Halt attacks immediately or face punishing air strikes. But U.S. President Barack Obama also gave the Libyan despot an out, promising the gathering warplanes won't bomb him into regime change if he quits attacking rebel-held cities.
The unpredictable and ruthless Libyan leader declared a unilateral ceasefire hours before Mr. Obama's ultimatum, but Western leaders on the verge of launching bombing runs were unconvinced, demanding proof the guns had fallen silent and not simply the slippery assurances of Colonel Gadhafi.
At air bases from Spain to Sicily and in Egypt's Western desert, warplanes gathered. In the Mediterranean, warships and submarines readied cruise missiles. Overhead, American spy satellites and NATO surveillance planes readying for the launch orders that might be only hours away.
"Moammar Gadhafi has a choice," Mr. Obama warned. "The ceasefire must be implemented immediately. That means all attacks against civilians must stop. Gadhafi must stop his troops from advancing on Benghazi; pull them back from Ajdabiya, Misurata and Zawiya."
If Col. Gadhafi meets that ultimatum, it would leave him ruling most of Libya, including the capital Tripoli, all of the major oil fields and the key oil-shipment ports.
"I also want to be clear about what we will not be doing," Mr. Obama said, essentially laying out the terms under which Col. Gadhafi could remain in power. "The United States is not going to deploy ground troops into Libya, and we are not going to use force to go beyond a well-defined goal, specifically the protection of civilians in Libya."
No deadline was set, either by Mr. Obama or in the UN Security Council resolution.
In several rebel-held cities, notably Misurata and Ajdabiya, which controls the approaches to the breakaway stronghold of Benghazi, Libyans reached by telephone said heavy shelling by forces loyal to Col. Gadhafi had continued throughout Friday.
British and French leaders - who spearheaded the effort to sanction international military intervention - voiced grave doubts about Libyan sincerity.
U.S. Secretary of State Hillary Clinton said the powers readying air strikes wouldn't be "impressed by words." We need to "see actions on the ground, and that is not yet at all clear."
Long regarded as a terrorist-sponsoring pariah and something of a thuggish buffoon for his bizarre lifestyle, the West welcomed Col. Gadhafi back into the community of nations after he gave up a nuclear-weapons program five years ago. The Libyan leader is now vilified again for his brutal crackdown on the uprising seeking to oust him.
"Left untouched," Mr. Obama said, "we have every reason to believe Gadhafi would commit atrocities against his people."
"Here's why this matters to us," the President said, as the U.S. military, already stretched thin with a major ongoing war in Afghanistan and still heavily committed to Iraq, girded for action in yet another Muslim country. "The entire region could be destabilized, endangering many of our allies and partners."
The usually vociferous and threatening Col. Gadhafi - after weeks of vowing death and destruction to his enemies and outsiders who dared interfere - was silent.
Still, the Security Council decision to allow nations to wage limited war against Col. Gadhafi's brutal efforts to reassert control over rebel cities will create a difficult precedent.
In Yemen, Bahrain and Syria, repressive governments were suppressing pro-democracy protesters with violence.
Mr. Obama seemed content to let the French and British lead and determined that at least some Arab nations commit warplanes to the gathering military effort. But the U.S. military, which dwarfs the combined air forces and navies of all of the nations so far committed to the Libyan operation, remains central to its success.
"Change in the region will not, and cannot, be imposed by the United States or any foreign power," Mr. Obama said.
But cheering Benghazis, who thronged Libya's second city with celebratory festivities after the Security Council vote, see the prospect of outside military intervention as deliverance.
French President Nikolas Sarkozy will host a "coalition of the willing" meeting Saturday in Paris. Canada, Britain, Spain, the United States, Denmark, Portugal, Italy, and the Arab League were all expected to attend. Germany - which irked its European partners by opting out of military action - wasn't invited.
By the time leaders meet, British and French warplanes may have already probed Libyan air space, testing whether Col. Gadhafi's radar stations and surface-to-air missiles sites will dare "lock on" and invite destruction.
From Canada, six CF-18 fighter-bombers, last in action more than a decade ago bombing Serb positions in Kosovo, will deploy to a southern European allied air base. Italy promised use of air bases. Denmark was sending six warplanes. A French aircraft carrier was preparing for sea. A U.S. naval battle group - not including an aircraft carrier - was due to leave Norfolk, Va., to reinforce American warships already in the Mediterranean.