Cruising a Wal-Mart in Clayton County, Ga., with Sgt. Russell Haney of U.S. Army recruiting, it would be easy to think most Americans are aching to serve Uncle Sam.

Almost every teenager or 20-something he hails, in his cheery Tennessee drawl, appears tempted by his offer. Lemeanfa, a 19-year-old former football star, says he is halfway sold on it; Dseanna, an 18-year-old shopper, says she is too, provided she won't have to go to war.

Serving in the coffee shop, Archel and Lily, a brother and sister from the U.S. Virgin Islands, listen greedily to the benefits the recruiting sergeant reels off. "You don't want a job, you want a career!" he tells them, as a passer-by thrusts a packet of cookies into his hands, to thank him for his service.

Southern, poorer than the national average, mostly black and with long-standing ties to the Army, the inhabitants of Clayton County are among the army's likeliest recruits. Last year they furnished it with more soldiers than most of the rest of the greater Atlanta area put together. Yet Haney's battalion, which is responsible for it, still failed to make its annual recruiting target.

In the financial year that ended on Sept. 30, America's four armed services — Army, Navy, Air Force, Marines — aimed to recruit 177,000 people, mainly from among the 21 million Americans aged 17-21.

Yet all struggled, and the Army, which accounted for nearly half that target, made its number, at great cost and the 11th hour, only by cannibalizing its store of recruits for the current year. It failed by 2,000 to meet its target of 17,300 recruits for the Army Reserve, which is becoming more important to national security as the full-time Army shrinks from a recent peak of 566,000 to a projected 440,000 by 2019 — its lowest level since World War II.

That is part of a long-standing trend: a growing disconnect between American society and the armed forces that claim to represent it, which has many causes, starting with the ending of the draft in 1973. Ever since, military experience has been steadily fading from American life.

Seasonal factors, including a strengthening labor market and negative media coverage of the wars in Iraq and Afghanistan, have widened the gulf. So have the dismal standards of education and physical fitness that prevail in modern American society. At a time of postwar introspection, these factors raise two big questions. The first concerns America's ability to hold to account a military sector its leaders feel bound to applaud, but no longer competent to criticize. The second question raised by the civil-military disconnect is similarly fundamental: It concerns America's future ability to mobilize for war.

During the Korean War, around 70 percent of draft-age American men served in the armed forces; during Vietnam, the unpopularity of the conflict and ease of draft-dodging ensured that only 43 percent did.

These days, even if every young American wanted to join up, less than 30 percent would be eligible to. Of the starting 21 million, around 9.5 million would fail a rudimentary academic qualification, either because they had dropped out of high school or, typically, because most young Americans cannot do tricky sums without a calculator.

Of the remainder, 7 million would be disqualified because they are too fat, or have a criminal record, or tattoos on their hands or faces. According to Haney, about half the high-school students in Clayton County are inked somewhere or other; according to his boss, Lt. Col. Tony Parilli, a bigger problem is simply that "America is obese."

That leaves 4.5 million young Americans eligible to serve, of whom only around 390,000 are minded to, provided they do not get snapped up by a college or private firm instead — as tends to happen to the best of them.

With the annual exception of a few hundred sons and daughters of retired officers, America's elite has long since turned its nose up at military service. Well under 10 percent of army recruits have a college degree.

The pool of potential recruits is too small to meet America's, albeit shrunken, military needs; especially, as now, when the unemployment rate dips below 6 percent. This leaves the army, the least-favored of the four services, having either to drop its standards or entice those not minded to serve with generous perks.

After it failed to meet its recruiting target in 2005, a time of high employment and bad news from Baghdad, it employed both strategies zealously. To sustain what was, by historical standards, only a modest surge in Iraq, around 2 percent of army recruits were accepted despite having failed to meet academic and other criteria. Meanwhile, the cost of the Army's signing-on bonuses ballooned unsustainably, to $860 million in 2008 alone.

That figure has since fallen, as part of a wider effort to peg back the personnel costs that consume around a quarter of the defense budget. Yet the remaining sweeteners are still generous: The Army's pay and allowances have risen by 90 percent since 2000.

In a role-play back at Haney's recruiting station, your correspondent, posing as an aimless school-leaver, asked what the Army could offer him. The answer, besides the usual bed, board and medical insurance, included $78,000 in college fees, some of which could be transferred to a close relative; professional training, including for 46 jobs that still offer a fat signing-on bonus; and post-service career advice.

It is a good offer, especially set against the bad jobs and wage stagnation prevalent among the Americans it is mostly aimed at. That the Army is having trouble selling it is partly testament to the effects on public opinion of recent wars.

Most young Americans associate the Army with "coming home broken, physically, mentally and emotionally," says James Ortiz, the Army's director of marketing.

The result is that America may be unable, within reasonable cost limits and without reinstituting the draft, to raise the much bigger Army it might need for such wars.

Copyright 2013 The Economist Newspaper Limited, London. All Rights Reserved. Reprinted with permission.