It would be based, at least initially, on the West Coast, probably at Vandenberg Air Force Base.

Under the Obama plan, the Prompt Global Strike warhead would be mounted on a long-range missile to start its journey toward a target. It would travel through the atmosphere at several times the speed of sound, generating so much heat that it would have to be shielded with special materials to avoid melting. (In that regard, it is akin to the problem that confronted designers of the space shuttle decades ago.)

But since the vehicle would remain within the atmosphere rather than going into space, it would be far more maneuverable than a ballistic missile, capable of avoiding the airspace of neutral countries, for example, or steering clear of hostile territory. Its designers note that it could fly straight up the middle of the Persian Gulf before making a sharp turn toward a target.

The Pentagon hopes to deploy an early version of the system by 2014 or 2015. But even under optimistic timetables, a complete array of missiles, warheads, sensors and control systems is not expected to enter the arsenal until 2017 to 2020, long after Mr. Obama will have left office, even if he is elected to a second term.

The planning for Prompt Global Strike is being headed by Gen. Kevin P. Chilton of the Air Force, the top officer of the military’s Strategic Command and the man in charge of America’s nuclear arsenal. In the Obama era — where every administration discussion of nuclear weapons takes note of Mr. Obama’s commitment to moving toward “Global Zero,” the elimination of the nuclear arsenal — the new part of General Chilton’s job is to talk about conventional alternatives.

In an interview at his headquarters at Offutt Air Force Base, General Chilton described how the conventional capability offered by the proposed system would give the president more choices.

“Today, we can present some conventional options to the president to strike a target anywhere on the globe that range from 96 hours, to several hours maybe, 4, 5, 6 hours,” General Chilton said.

That would simply not be fast enough, he noted, if intelligence arrived about a movement by Al Qaeda terrorists or the imminent launching of a missile. “If the president wants to act on a particular target faster than that, the only thing we have that goes faster is a nuclear response,” he said.