National Guard units are the nation's citizen soldiers. Their members come from all walks of life, from machine shop and factory, from field and schoolhouse and many occupations besides. They are the linear descendants of the few patriots who once mustered at Lexington to oppose the forces of a tyrannical empire.

They were the Army then. They are merely indispensable to the Army now. Instead of being the standing reserve, these latter-day militia men (and women) are being sent into battle in places like Iraq and Afghanistan. The distinction between professional soldier and civilian soldier hardly exists.

This is happening to an extent that is historic in its sweep. As the Post-Gazette reported Saturday, about 2,400 soldiers in the Pennsylvania Army National Guard have been called up for duty in Iraq, about 15 percent of the total. It was one of the largest mobilizations since World War II.

The Guard members are likely to do tours of 18 months. As other Americans see this happening, and mouth the daily platitude "we support our troops," they can justify this unequal sacrifice by saying that these good people signed up to do it.

Not really. Eighteen months away from home and family is surely more than most citizen soldiers ever thought they were getting into. And yet, to their great credit and honor, they salute and get on with the job. The military presence in Iraq is belatedly being increased to 150,000, and Defense Secretary Donald Rumsfeld, who underestimated what was needed in the first place, now says that U.S. forces can be withdrawn within four years.

That begs the question: How long can so much of a burden be placed on the comparative few so that the mass of Americans can rest easy, free of duty and care? Reports from across the nation suggest the Army is now calling up the old and the infirm as well as the able-bodied to keep up the numbers. Eventually, the strain on those who serve -- the sheer unfairness of the situation -- will take its toll.

A few warning signs are there already. A few soldiers have sued to challenge the notorious "stop-loss" policy that allows the Pentagon to keep people in the military beyond their normal discharge dates. Whatever the legal merits of these cases, they are a cry of frustration.

The fact is that U.S. forces are drastically overextended and the time has come to rethink deployment policies. Supporting the troops also means being fair in what the nation asks of them.