The concrete goes on forever, vanishing into the noonday glare, 2 million cubic feet of it, a mile-long slab that's now the home of as many as 120 US helicopters, a ''heli-park" as good as any back in the States.

At another giant base, Asad in Iraq's western desert, the 17,000 troops and workers come and go in a kind of bustling American town, with a Burger King and Pizza Hut, a car dealership, stop signs, traffic regulations, and young bikers clogging the roads.

At a third hub down south, Tallil, they're planning a new mess hall, one that will seat 6,000 hungry airmen and soldiers for chow.

Are the Americans here to stay? Air Force mechanic Josh Remy is sure of it as he looks around Balad.

''I think we'll be here forever," said the 19-year-old airman from Wilkes-Barre, Pa.

The Iraqi people suspect the same. Strong majorities tell pollsters they would like to see a timetable for US troops to leave, but believe Washington plans to keep military bases in their country.

The question of America's future in Iraq looms larger as the US military enters the fourth year of its war here. On Tuesday, President Bush said the decision of when to remove US troops rests with ''future presidents and future governments in Iraq."

Ibrahim al-Jaafari, interim prime minister, has said he opposes permanent foreign bases. A wide range of American opinion is against them, as well. Such bases would be a ''stupid" provocation, said General Anthony Zinni, former US Mideast commander and a critic of the original invasion.

But events, in explosive situations like Iraq's, can turn ''no" into ''maybe" and even ''yes."

The Shi'ite Muslims, ascendant in Baghdad, might decide they need long-term US protection against insurgent Sunni Muslims. Washington might take the political risks to gain a strategic edge -- in its confrontation with next-door Iran, for example.

The US ambassador to Iraq, Zalmay Khalilzad, and other US officials disavow any desire for permanent bases. But long-term access, as at other American bases abroad, is different from ''permanent," and the official US position is carefully worded.

Lieutenant Commander Joe Carpenter, a Pentagon spokesman on international security, said it would be ''inappropriate" to discuss future basing until a new Iraqi government is in place, expected in the coming weeks.

Less formally, Defense Secretary Donald Rumsfeld, asked about ''permanent duty stations" by a Marine during an Iraq visit in December, allowed that it was ''an interesting question." He said it would have to be raised by the incoming Baghdad government, if ''they have an interest in our assisting them for some period over time."

In Washington, Iraq scholar Phebe Marr finds the language intriguing. ''If they aren't planning for bases, they ought to say so," she said. ''I would expect to hear 'No bases.' "

Right now what is heard is the pouring of concrete.

For 2005-06, Washington has authorized or proposed almost $1 billion for US military construction in Iraq, as American forces consolidate at Balad, known as Anaconda, and a handful of other installations, big bases under the old regime.

They have already pulled out of 34 of the 110 bases they were holding last March, said Major Lee English of the US command's Base Working Group, planning the consolidation.

''The coalition forces are moving outside the cities while continuing to provide security support to the Iraqi security forces," English said.

The move away from cities, perhaps eventually accompanied by US force reductions, will lower the profile of American troops, frequent targets of roadside bombs on city streets. Officers at Asad Air Base, 10 desert miles from the nearest town, say it hasn't been hit by insurgent mortar or rocket fire since October.

Asad will become even more isolated. The proposed 2006 supplemental budget for Iraq operations would provide $7.4 million to extend the no man's land and build new security fencing around the base, which at 19 square miles is so large that many assigned there take the Yellow or Blue bus routes to get around the base, or buy bicycles at a PX jammed with customers.

The latest budget also allots $39 million for new airfield lighting, air traffic control systems, and upgrades allowing Asad to plug into the Iraqi electricity grid -- a typical sign of a long-term base.

At Tallil, besides the new $14 million dining facility, Ali Air Base is to get, for $22 million, a double perimeter security fence with high-tech gate controls, guard towers, and a moat -- in military parlance, a ''vehicle entrapment ditch with berm."

Here at Balad, the former Iraqi Air Force academy 40 miles north of Baghdad, the two 12,000-foot runways have become the logistics hub for all US military operations in Iraq, and major upgrades began last year.

Army engineers say 31,000 truckloads of sand and gravel fed nine concrete-mixing plants on Balad, as contractors laid a $16 million ramp to park the Air Force's huge C-5 cargo planes; an $18 million ramp for workhorse C-130 transports; and the vast, $28 million main helicopter ramp, the length of 13 football fields, filled with attack, transport, and reconnaissance helicopters.

Turkish builders are pouring tons more concrete for a fourth ramp beside the runways, for medical-evacuation and other aircraft on alert. And $25 million was approved for other ''pavement projects," from a special road for munitions trucks to a compound for special forces.

The chief Air Force engineer here, Lieutenant Colonel Scott Hoover, is also overseeing two crucial projects to add to Balad's longevity: equipping the two runways with new permanent lighting, and replacing a weak 3,500-foot section of one runway.

Once that's fixed, ''we're good for as long as we need to run it," Hoover said. Ten years? he was asked. ''I'd say so."

Away from the flight lines, among traffic jams and freshly planted palms, life improves on 14-square-mile Balad for its estimated 25,000 personnel, including several thousand American and other civilians.

They've inherited an Olympic-sized pool and a chandeliered cinema from the Iraqis. They can order their favorite Baskin-Robbins flavor at ice cream counters in five dining halls, and cut-rate Fords, Chevys, or Harley-Davidsons, for delivery at home, at a PX-run ''dealership." On one recent evening, not far from a big 24-hour gym, airmen hustled up and down two full-length, lighted outdoor basketball courts as F-16 fighters thundered overhead.

''Balad's a fantastic base," Brigadier General Frank Gorenc, the Air Force's tactical commander in Iraq, said at his headquarters here.

Could it host a long-term US presence?

''Eventually it could," said Gorenc, commander of the 332d Air Expeditionary Wing. ''But there's no commitment to any of the bases we operate, until somebody tells me that."

In the counterinsurgency fight, Balad's central location enables strike aircraft to reach targets in minutes. And in the broader context of reinforcing the US presence in the oil-rich Mideast, Iraq bases are preferable to aircraft carriers in the Persian Gulf, said a longtime defense analyst.