US Defense Secretary Robert Gates called for the military to develop an enduring capacity to fight "irregular" wars, and to rethink its reliance on ever more costly high-tech weapons.

Writing in Foreign Affairs quarterly, Gates said the United States needs "a military whose ability to kick down the door is matched by its ability to clean up the mess and even rebuild the house afterward."

"What is dubbed the war on terror is, in grim reality, a prolonged, worldwide irregular campaign -- a struggle between the forces of violent extremism and those of moderation," he wrote.

Published just days after president-elect Barack Obama asked Gates to stay on at the Pentagon, his article coincided with a new Defense Department directive that puts the fight against terrorism and guerrilla warfare on the same footing as conventional warfare for the first time.

"It is DoD (Department of Defense) policy to recognize that IW (irregular warfare) is as strategically important as traditional warfare," the directive states.

The Pentagon directive defined irregular warfare as encompassing counter-terrorism operations, guerrilla warfare, foreign internal defense, counter-insurgency and stability operations.

Gates used the article to push irregular warfare to the center of a military institution that historically has preferred to focus on fighting big, conventional wars.

Outside of the special forces community and some dissident colonels, there has never been strong institutional support for irregular warfare.

"Support for conventional modernization programs is deeply embedded in the Defense Department's budget, in its bureaucracy, in the defense industry, and in Congress," Gates said.

"My fundamental concern is that there is not commensurate institutional support -- including in the Pentagon -- for the capabilities needed to win today's wars and some of their likely successors."

Gates also suggested that, given limits on resources, the military's tendency to procure more capable but fewer and more costly weapons systems has "perhaps reached a point of diminishing returns."

"In recent years, these platforms have grown ever more baroque, have become ever more costly, are taking longer to build, and are being fielded in ever-dwindling quantities," he said.

Gates argued that US conventional military dominance could allow the country to risk fielding cheaper, less-sophisticated weaponry more rapidly.

"The time has come to consider whether the specialized, often relatively low-tech equipment well suited for stability and counterinsurgency missions is also needed," he said.

US military chiefs have warned, however, that US ground forces have been so wrapped up in counter-insurgency warfare in Iraq and Afghanistan that they are ill-prepared for conventional war.

Gates acknowledged that the military would be hard put to fight another major war right now, but he said the risk was "prudent and manageable" because there remains enough untapped air and naval power to deter and punish aggressors.

The Pentagon directive instructs the Defense Department to develop capabilities to:

-- identify and prevent or defeat irregular threats from state and non-state actors

-- extend US reach into denied areas and uncertain environments by operating with and through indigenous foreign forces

-- train, advise and assist foreign security forces and partners

-- support a foreign government or population threatened by irregular adversaries

-- create a safe, secure environment in fragile states.

Separately, the Joint Forces Command released a report Thursday that warns that the US military must be prepared for a full range of conflicts over the next 25 years.

"Nuclear and major regular war may represent the most important conflicts the Joint Force could confront, but they remain the least likely," the report said.

"Irregular wars are more likely, and winning such conflicts will prove just as important to the protection of America's vital interests and the maintenance of global stability," it said.

The report said it would be difficult for US forces to prepare for such a wide range of threats.

But it added, "The difficulties involved in training to meet regular and nuclear threats must not push preparations to fight irregular war into the background, as occurred in the decades after the Vietnam War."