Early last summer, United States Defense Secretary Donald Rumsfeld approved a top secret "Interim Global Strike Alert Order'' directing the military to assume and maintain readiness to attack hostile countries that are developing weapons of mass destruction, specifically Iran and North Korea.

Two months later, Lieutenant-General Bruce Carlson, commander of the 8th Air Force, told a reporter that his fleet of B-2 and B-52 bombers had changed its way of operating so that it could be ready to carry out such missions. "We're now at the point where we are essentially on alert,'' Carlson said in an interview with the Shreveport Times.

"We have the capacity to plan and execute global strikes.''

Carlson said his forces were the US Strategic Command's "focal point for global strike'' and could execute an attack "in half a day or less.''

In the secret world of military planning, global strike has become the term to describe a specific pre-emptive attack. When military officials refer to global strike, they stress its conventional elements. Surprisingly, however, global strike also includes a nuclear option, which runs counter to traditional US notions about the defensive role of nuclear weapons.

The official US position on the use of nuclear weapons has not changed. Since the end of the Cold War, the United States has taken steps to de-emphasize the importance of its nuclear arsenal.

The Bush administration has said it remains committed to reducing its nuclear stockpile while keeping a credible deterrent against other nuclear powers. Administration and military officials have stressed this continuity in testimony over the past several years before various congressional committees.

But a confluence of events, beginning with the September 11, 2001, attacks and the president's forthright commitment to the idea of pre-emptive action to prevent future attacks, has set in motion a process that has led to a fundamental change in how the US military might respond to certain possible threats.

Understanding how Washington got to this point, and what it might mean for US policy, is particularly important now - with the renewed focus last week on Iran's nuclear intentions and on speculation that North Korea is ready to conduct its first test of a nuclear weapon.

Global strike has become one of the core missions for the Omaha-based Strategic Command, or Stratcom.

Once, Stratcom oversaw only the nation's nuclear forces; now it has responsibility for overseeing a global strike plan with both conventional and nuclear options.

President George W Bush spelled out the definition of a "full-spectrum'' global strike in a January 2003 classified directive, describing it as "a capability to deliver rapid, extended range, precision kinetic (nuclear and conventional) and non-kinetic effects in support of theater and national objectives.''

This blurring of the nuclear/conventional line, wittingly or unwittingly, could heighten the risk that the nuclear option will be used.

Exhibit A may be the Stratcom contingency plan for dealing with "imminent'' threats from countries such as North Korea or Iran, formally known as Conplan 8022-02.

Conplan 8022 is different from other war plans in that it posits a small-scale operation and no "boots on the ground.''

The typical war plan encompasses an amalgam of forces - air, ground, sea - and takes into account the logistics and political dimensions needed to sustain those forces in protracted operations. All these elements generally require significant lead time to be effective.

Existing Pentagon war plans, developed for specific regions or "theaters,'' are essentially defensive responses to invasions or attacks. The global strike plan is offensive, triggered by the perception of an imminent threat and carried out by presidential order.

Conplan 8022 anticipates two sce-narios. The first is a response to a specific and imminent nuclear threat, say in North Korea.

A quick-reaction, highly choreographed strike would combine pinpoint bombing with electronic warfare and cyberattacks to disable a North Korean response, with commandos operating deep in enemy territory, perhaps even to take possession of the nuclear device.

The second scenario involves a more generic attack on an adversary's weapons-of-mass-destruction infrastructure.

Assume, for argument's sake, that Iran announces it is mounting a crash program to build a nuclear weapon. A multidimensional bombing (kinetic) and cyberwarfare (non-kinetic) attack might seek to destroy Iran's program, and special forces would be deployed to disable or isolate underground facilities.

By employing all of the tricks in the US arsenal to immobilize an enemy country - turning off the electricity, jamming and spoofing radars and communications, penetrating computer networks and garbling electronic commands - global strike magnifies the impact of bombing by eliminating the need to physically destroy targets that have been disabled by other means.

The inclusion, therefore, of a nuclear weapons option in Conplan 8022 - a specially configured earth-penetrating bomb to destroy deeply buried facilities, if any exist - is particularly disconcerting.

The global strike plan holds the nuclear option in reserve if intelligence suggests an "imminent'' launch of an enemy nuclear strike on the United States or if there is a need to destroy hard-to-reach targets.

It is difficult to imagine a US president ordering a nuclear attack on Iran or North Korea under any circumstance. Yet as global strike contingency planning has moved forward, so has the nuclear option.

Global strike finds its origins in pre-Bush administration Air Force thinking about a way to harness American precision and stealth to "kick down the door'' of defended territory, making it easier for (perhaps even avoiding the need for) follow-on ground operations.

The events of 9/11 shifted the focus of planning. There was no war plan for Afghanistan on the shelf, not even a generic one.

In Afghanistan, the synergy of conventional bombing and special operations surprised everyone. But most importantly, weapons of mass destruction became the US government focus.

It is not surprising, then, that barely three months after that earth-shattering event, the Pentagon's quadrennial Nuclear Posture Review assigned the military and Stratcom the task of providing greater flexibility in nuclear attack options against Iraq, Iran, North Korea, Libya, Syria and China.

The Air Force's global strike concept was taken over by Stratcom and made into something new.

This was partly in response to the realization that the military had no plans for certain situations. The possibility that some nations would acquire the ability to attack the US directly with a WMD, for example, had clearly fallen between the command structure's cracks.

For example, the Pacific Command in Hawaii had many war plans on its shelf to respond to a North Korean attack on South Korea, including some with nuclear options.

But if North Korea attacked the United States directly or, more to the point, if the US intelligence network detected evidence of preparations for such an attack, Pacific Command didn't have a war plan in place.

In May 2002, Rumsfeld issued an updated Defense Planning Guidance that directed the military to develop an ability to undertake "unwarned strikes ... [to] swiftly defeat from a position of forward deterrence.''

The post-9/11 National Security Strategy, published in September 2002, codified pre-emption, stating that the United States must be prepared to stop "rogue states and their terrorist clients before they are able to threaten or use weapons of mass destruction against the United States and our allies.''

"We cannot let our enemies strike first,'' Bush declared in the National Security Strategy document.

Stratcom established an interim global strike division to turn the new preemption policy into an operational reality.

In December 2002, Admiral James Ellis Jr, then Stratcom's head, told an Omaha business group that his command had been charged with developing the capability to strike anywhere in the world within minutes of detecting a target. Ellis posed the following question to his audience: "If you can find that time-critical, key terrorist target or that weapons-of-mass-destruction stockpile, and you have minutes rather than hours or days to deal with it, how do you reach out and negate that threat to our nation half a world away?''

Conplan 8022-02 was completed in November 2003, putting in place for the first time a pre-emptive and offensive strike capability against Iran and North Korea.

In January 2004, Ellis certified Stratcom's readiness for global strike to the defense secretary and the president.

At Ellis' retirement ceremony in July, General Richard Myers, chairman of the Joint Chiefs of Staff, told an Omaha audience that "the president charged you to `be ready to strike at any moment's notice in any dark corner of the world' [and] that's exactly what you've done.''

As US military forces have become bogged down in Afghanistan and Iraq, the attractiveness of global strike planning has increased in the minds of many in the military. Stratcom planners, recognizing that US ground forces are already overcommitted, say that global strike must be able to be implemented "without resort to large numbers of general purpose forces.''

When one combines the doctrine of pre-emption with a "homeland security'' ethic that concludes that only hyper-vigilance and readiness stand in the way of another 9/11, it is pretty clear how global strike ended up where it is.

The 9/11 attacks caught the country unaware and the natural reaction of contingency planners is to try to eliminate surprise in the future. The Nuclear Posture Review and Rumsfeld's classified Defense Planning Guidance both demanded more flexible nuclear options.

Global strike thinkers may believe they have found a way to keep the nuclear genie in the bottle but they are also having to cater to a belief on the part of those in government's inner circle who have convinced themselves that the gravity of the threats demands that the United States does not engage in any protracted debate, that it prepares for the worst and hopes for the best.

Though the official Washington mantra has always been "we don't discuss war plans,'' here is a real life predicament that cries out for debate: In classic terms, military strength and contingency planning can dissuade an attacker from mounting hostile actions by either threatening punishment or demonstrating through preparedness that an attacker's objectives could not possibly be achieved.

The existence of a nuclear capability, and a secure retaliatory force, moreover, could help to deter an attack - that is, if the threat is credible in the mind of the adversary.

But the global strike contingency plan cannot be a credible threat if it is not publicly known. And though Conplan 8022 suggests a clean, short-duration strike intended to protect American security, a pre-emptive surprise attack (let alone one involving a nuclear weapon option) would unleash a multitude of additional and unanticipated consequences. So, on both counts, why aren't we talking about it?

THE WASHINGTON POSTWilliam Arkin, who writes frequently about military affairs, is the author of Code Names: Deciphering US Military Plans, Programs and Operations in the 9/11 World (Steerforth).