WASHINGTON  In late 2009, the Obama administration was leaning on Col. Muammar el-Qaddafi and his son Seif to allow the removal from Libya of the remnants of the country’s nuclear weapons program: casks of highly enriched uranium.

Meeting with the American ambassador, Gene A. Cretz, the younger Qaddafi complained that the United States had retained “an embargo on the purchase of lethal equipment” even though Libya had turned over more than $100 million in bomb-making technology in 2003. Libya was “fed up,” he told Mr. Cretz, at Washington’s slowness in doling out rewards for Libya’s cooperation, according to cables released by WikiLeaks.

Today, with father and son preparing for a siege of Tripoli, the success of a joint American-British effort to eliminate Libya’s capability to make nuclear and chemical weapons has never, in retrospect, looked more important.

Senior administration officials and Pentagon planners, as they discuss sanctions and a possible no-flight zone to neutralize the Libyan Air Force, say that the 2003 deal removed Colonel Qaddafi’s biggest trump card: the threat of using a nuclear weapon, or even just selling nuclear material or technology, if he believed it was the only way to save his 42-year rule. While Colonel Qaddafi retains a stockpile of mustard gas, it is not clear he has any effective way to deploy it.