Washington ? Armies are fragile institutions, and for all their might, easily broken.

It took the better part of 20 years to rebuild the Army from the wreckage of Vietnam. With the hard work of a generation of young officers, blooded in Vietnam and determined that the mistake would never be repeated, a new Army rose Phoenix-like from the ashes of the old, now perhaps the finest Army in history.

In just over two years, Secretary of Defense Donald H. Rumsfeld and his civilian aides have done just about everything they could to destroy that Army.

How will they explain to history the mistakes that threaten to weaken a great nation even as it seeks ways to win the war on terrorism it has declared?

How do you break an army?

• You can work it to death.

Under Rumsfeld, by next spring 30 of the Army’s 33 combat brigades will either be in Iraq or on their way home from Iraq. Some of them will come home from Iraq and head almost immediately to Afghanistan or Bosnia or South Korea or the Sinai Desert. More than 20,000 Army Reservists and National Guardsmen will be finishing one-year tours in Iraq, and thousands more will be called up to do their year. How many will be willing to re-enlist if they’re faced with endless deployments on thankless missions in the far reaches of empire?

• You can neglect its training and education.

With an operations tempo this high, there’s little time for units to do much more than repair their equipment and send their soldiers home on leave with long-neglected families before it’s time to deploy again.

There’s no time for divisions to rotate to the National Training Center at Fort Irwin, Calif., to maneuver their Abrams tanks and Bradleys and train to win the wars. There’s no time for non-commissioned officers — the sergeants who are the backbone of any great army — to go to the schools they need to get better at their jobs and earn promotions.

The Army began to break in Vietnam when the senior NCO’s, the grizzled old sergeants who’d seen combat in World War II and Korea and survived one or two tours in Vietnam, were ordered back yet again and chose to retire instead. Or went back and were killed. In their place came 90-day wonders — young draftees selected straight out of basic training, run through a short course and shipped to Vietnam to be buck sergeant squad leaders.

• You can politicize the Army promotion system for three- and four-star generals.

Rumsfeld and his civilian aides such as Paul Wolfowitz and Douglas Feith and his military handmaidens have intruded deeply and harmfully into the way the services promote their leaders.

Where once the Army would send up its nominee for a vacant billet, now it must send up two or three candidates who must run the gantlet of personal interviews the Defense Department. Not just Rumsfeld, but all of his civilian experts who never wore a uniform. What hoops must the successful one jump through? Will it be the tough, bright candidate who’s unafraid to speak when he sees mistakes being made? Or will it be the buttoned-down, willow-in-the-wind, can-do yes-man? Your basic Oliver North?

• You can decide that you’ve discovered a newer, cheaper way of fighting and winning America’s wars.

Rumsfeld and company have em-braced, on the basis of fleeting success in Afghanistan and flawed success in Iraq, a theory that all that’s needed to win wars is air power and small bands of Special Operations troops. Stealth bombers and snake-eaters.

On the strength of this, they’ve refused all pleas for an urgently needed increase in the strength of an Army that has been whittled down to pre-World War II levels of 485,000 soldiers. They still deny that there’s a guerrilla war raging in Iraq, where 130,000 American soldiers are trying to keep the peace in a nation the size of California with 25 million people. Because reinforcement would be an admission that Rumsfeld and company were wrong in their belief that war would end quickly, their hand-picked Iraqi exiles would take over and the soldiers would come home in a few months.

Another defense secretary who could not admit he’d erred was Robert Strange McNamara, who, like Rumsfeld, was recruited from corporate America. By the time he did, it was too late.