It is best, some people would suggest, to think of the Pentagon as a dangerously obese man. He should not be allowed to gorge further; he should be taken away from the table. As for President Bush's five-year, $2.1trn defence spending deal, it may only force-feed the patient until he explodes.

"Too much money has enabled the Pentagon to avoid reform and transformation," says John Isaacs, defence analyst for the Washington think-tank Council for a Livable World. "What it really needs is a ten-year diet."

The standard objection on the European left to Bush's bonanza budget can be summarised thus: money for development = good; $48bn increase in defence spending = bad, very bad. Although many liberal critics in America think the same, some have a further complaint: namely, that the new budget is not just throwing a huge amount of money away on weaponry, but is throwing a huge amount away on the wrong kind of weaponry.

So how exactly does the world's only remaining superpower use $2.1trn in defence expenditure? What next for a country whose military budget was already six times larger than those of all the "axis of evil" countries combined - plus Cuba, Sudan and Syria thrown in for good measure? A weather machine?

What most critics agree on is the preponderance of heavy cold-war weaponry on the bill, at the expense of the Pentagon's proposed "transformation" - a strategy to reshape the army as a lighter, more deployable force.

The lion's share of this year's proposed $396.1bn defence budget will pay for controversial cold-war white elephants such as United Defense's Crusader howitzer, a tank-like vehicle designed to repel a Soviet invasion of Europe.

"The recent military build-up seems to have little to do with the actual threat," wrote the economist Paul Krugman in the New York Times, "unless you think al-Qaeda's next move will be a frontal assault by several heavily armoured vehicles."

Leaving obese men to one side, there is no better real-life metaphor for military waste than the Fort Hood army base in central Texas. "It's the Grand Canyon of armour power," wrote the journalist William Greider in his 1998 investigation of military economics, Fortress America: the American military and the consequences of peace.

Tucked in among Fort Hood's gentle prairie creeks and hills is one of the largest, deadliest and costliest displays of firepower on earth. Row upon row of tanks, trucks, missiles, helicopters and howitzers stretch in their hundreds, thousands, then tens of thousands, each pointing their way westwards into New Mexico, then infinity. "It's exhausting to behold," noted Greider, "but there's nothing else like it in the world."

Greider, a celebrated muckraker who has worked at Rolling Stone and the Nation, wasn't just writing copy for the US army, package holiday division. His interest in Fort Hood was not in the capability that rests there - more than 40 per cent of US army forces - but in how, for most of the time, and at a cost of millions, that was all it was doing: resting. In effect, Fort Hood's magnificent vistas represent nothing more than a dumping ground for the world's most advanced fighting technology. And if critics are correct about the waste in Bush's budget, those vistas will be getting wider and more magnificent as the years go on.

The base's thousands of M-1 Abrams tanks, for example - "Cadillacs with guns", the soldiers call them - were used to devastating effect during the Gulf war against Saddam Hussein's much inferior forces, but there is little place for them in this era of Afghanistan-style in-and-out bombings.

In fact, so devastating is this newer form of air warfare that it is little wonder the Bush administration is feeling so cocky about downing Saddam's regime right now. Following the Soviet crash in 1991, no state in the world has the same capacity for military research and design as the United States.

Since the cold war, Pentagon spending has always been based on the premise that it is better to be safe than sorry. Why build 1,000 nuclear warheads - enough to bring about a nuclear winter - when you can build 6,000? The city of Los Angeles may have 1.4 million "food-insecure" (Californian for "hungry") people in 2002, but the army still gets to spend $48bn this year on the Commanche, a modernised version of a helicopter that hasn't been useful since the Korean war.

In the air, the only way the US could come up against an enemy with a plane as powerful as the F-16 would be by selling it to them. But this hasn't stopped Bush approving $12bn for three further jets - the Joint Strike Fighter, the F-18 Super Hornet and the F-22 Raptor - and for a redesign of the F-16. And that's only this year's costs; in the next half-decade, spending on the four fighters will increase to hundreds of billions.

The Crusader howitzer, which got a $475.2m payout this year, is one of the most contentious weapons in the budget, and a good example of what's wrong with it. The system consists of two parts: the howitzer weapon itself, a cannon-like barrel that fires between ten and 12 rounds a minute over a distance of 40km; and the resupply vehicle, which carries it across the battlefield. Both weigh a svelte 40 tons, making Crusader one of the deadliest - but also heaviest and slowest - mobile artillery weapons in existence.

But does anyone need it? On current evidence, future conflicts will provide scant openings for a slowcoach like Crusader. The Pentagon says battles are more likely to be dispersed over a large area, using unmanned vehicles and swift bombing raids (or "drive-by shootings", as the army calls them). Speed is all nowadays: just imagine how long it would take Crusader to get to Baghdad - more than 7,000 miles from the US - and that depends on finding a plane big enough to carry it. At the moment, the only jets capable of doing the job are the C-5 and the mammoth C-15 (aka "the flying truck"), and neither is in plentiful supply.

"To call Crusader a white elephant is an insult to white elephants," says Conn Hallinan, a columnist on military affairs for the San Francisco Examiner. "You can't fit it in a plane, it breaks any bridge it crosses, and you couldn't get it to Afghanistan on a dare."

So how does an obsolete weapon like Crusader get approved, let alone built and deployed? Because it creates jobs and security for thousands of people - not least defence companies and congressmen. When Oklahoma Representative J C Watts said he was "obviously sold" on Crusader last year, it wasn't just because he felt the howitzer was a really decent idea. A factory in Elgin, in Watts's district, will build parts for Crusader, ensuring his constituents hundreds of jobs - and his campaign thousands of votes. And in 2000, Watts was given $6,250 in contributions by United Defense's parent company, the Carlyle Group.

In the past year alone, United Defense has donated $62,750 to Congress. That might be small beer compared to Lockheed Martin ($900,000) or Philip Morris ($1m), but as far as political bribes go, it's enough to get things moving. Scratch wherever there's a factory making Crusader parts, and you'll almost certainly find a donation to the local congressperson from either Carlyle or United Defense, even if it's a measly thousand bucks.

The trick is to spread weapons sub-contractors out into as many congressional districts across the US as possible; that way, if the contract for a weapon of ambiguous use is questioned in Congress, it helps no end if thousands of congressmen's constituents are building different parts of it.

No defence company will willingly cancel its own contract. So the decision as to whether a weapon should be kept rests with the Pentagon accounting offices. For a bureau that regularly "misplaces" hundreds of billions of dollars, this is rarely helpful.

A study of the Crusader is instructive. Two years ago, George W Bush claimed Crusader was "too heavy" and "not lethal enough", in effect rejecting the programme in favour of the transformation. In April 2001, an advisory panel appointed by the secretary of defence, Donald Rumsfeld, actually recommended that Congress cancel Crusader and a host of others like it. At that point, it appeared, nothing would save the system, not even the presence in the Carlyle Group of George Bush Sr.

All that changed after 11 September. No one wanted to be seen as anything so wimpish as a dove any more, so the Pentagon dutifully renewed almost all of its weapons contracts, including Crusader. With the new funding taken into account, government spending on the howitzer amounts to $1.8bn since 1994. That's a phenomenal price for a weapon that even the president doesn't want. Unfortunately, it's not the only one (see box).

"The aftermath of the 11th has compounded all the problems in military spending," Greider told the NS. "The Bush team decided the window provided by a 'wartime presidency' was too good to pass up. By creating a high state of national alarm over an open-ended threat, it justifies not only endless armaments, but also a newly empowered national-security state."

That isn't just fighting terrorism, Greider speculates - "It's empire building."

Five programmes the Pentagon could cut
Amount indicates 2003 spending only

1. V-22 Osprey ($26bn): This vertical take-off aircraft has killed 30 soldiers just in the testing phase.

2. DDG-51 Destroyer ($2.7bn): Designed during the cold war to fight the Soviet navy. Where are they now?

3. B-61 "Bunker Busting" Nuclear Bomb (undisclosed sum): Because what the world really needs now is a new nuclear weapon.

4. Anti-Ballistic Missile Defence ($11bn): More than 20 years in the making and it still won't work.

5. F-22 Raptor ($5.2bn): Little improvement on the F-16 - which has no peer in the world - and said to be less effective than the in-production F-18.