It used to be called simply "war." Armies lined up across a battlefield, navies projected forces over oceans, and air forces bombed industrial sites and other strategic targets. All of it clearly involved enemy nations, and, for most of US history, took place on other continents.
In the 21st century, say military experts, the United States is more likely to face what some are calling "post-modern terrorists" with no particular national ties, attacking Americans and American society directly. In that sense, this century may be said to have started with the Sept. 11 attacks on the Pentagon and the World Trade Center.
The urgent questions now are: How far along is the US in meeting this new challenge? How much has been learned since a generation of officers, seared by their early experience in Vietnam, climbed the career ladder to become today's generals and admirals?
US officials acknowledge the new complexities and dangers. "All of us are having to adjust the way we are thinking about this new war," Joint Chiefs of Staff Chairman Gen. Richard Myers said at a recent Pentagon briefing. Last week, President Bush acknowledged the need to "configure our military ... so that we more effectively respond to asymmetrical responses from terrorist organizations."
But while US military forces have moved in the direction of preparing for unconventional warfare - or "asymmetric conflict," to use the current jargon - the changes have been more incremental than fundamental.
Until last month, the Pentagon spent most of its annual budget of more than $300 billion training and equipping to fight two major conventional wars at a time.
"At root, the 'American way of war' remains focused on a paradigm variously known as attrition, second-generation, or Industrial Age warfare," observes the Center for Defense Information, a private organization here headed by retired senior military officers. "This style of war-fighting tends to be linear and slow moving, relying on masses of men and material."
This is not a new criticism. Some 20 years ago, a group of young lawmakers (Al Gore and Newt Gingrich among them) formed the Military Reform Caucus on Capitol Hill to look at how the US military could better prepare for future wars. Meanwhile, as the cold war ended, the nation turned more toward defending its interests abroad (as in the Gulf War) or to taking the lead in settling regional conflicts (as in the Balkans). At the same time, terrorist attacks against US targets - mostly abroad - came with troubling regularity.
Now that those attacks have been directed at the homeland, military experts warn, the US - now the lone superpower and therefore more likely to be seen as either protector or adversary - is likely to find itself drawn into regional conflicts with connections to terrorist groups headed by "stateless actors"
In such cases, warns former Afghan Army Col. Ali Ahmad Jalali, "armed struggle is not an instrument of a clearly defined policy, but a means for open-ended gains in a volatile environment." (Mr. Jalali, a top planner with the Afghan resistance following the 1979 Soviet invasion, is now chief of Farsi language broadcasting for Voice of America.) "The trend defies classic norms of warfare and widely accepted military concepts," writes Jalali in Parameters, a War College quarterly.
It is now generally acknowledged that those norms and concepts will have to change to meet the challenge of international terrorism. This war "is not, as were previous wars, being fought over territory, nor is it a war of nation against nation," writes British military historian John Keegan in the Los Angeles Times this week. "The enemy is difficult to identify, much less conquer. And technological innovations have given birth to an entirely new set of weapons, which demand an entirely new set of responses."
Among the responses favored by military reformers: less emphasis on hardware (especially "big-ticket items" such as tanks, ships, and planes favored by weapons-makers and their congressional supporters) and more emphasis on smaller, more agile units, unit cohesion, and the initiative of individual commanders.
"Understand that success in conflict depends most upon people, then ideas, and least upon hardware," says Marcus Corbin, a senior analyst with the Center for Defense Information.
"Fix fraying leadership and cohesion in the military," he says, "in part by ending constant personnel rotation among units, halting the system of premature discharging of mid-level officers, and training and empowering officers to exercise more initiative. End a fixation on complex hardware, which is not only unreliable and expensive, but also creates complex bureaucracies to build, deploy, operate, supply, and fix it - bureaucracies that are unsuited to exercising the most important components of third- and fourth-generation warfare strategy: agility, quickness, flexibility, responsiveness, creativity, initiative."
It's a tall order - one that the Pentagon is aware of and in some ways already is following. But as senior officials from Defense Secretary Donald Rumsfeld on down acknowledge, much remains to be done.
"The world changed on September 11," says Keegan. "Now the nature of war must change to keep up with it."