A pair of U.S. Air Force F-15E Strike Eagles fly over northern Iraq early in the morning of Sept. 23, 2014, after conducting airstrikes in Syria.

WASHINGTON (MCT) — The air war in Syria and Iraq has already cost nearly $1 billion and ultimately could cost as much as $22 billion per year if a large ground force is deployed to the region, according to an analysis by the Center for Strategic and Budgetary Assessments.

The study, due to be released Monday, shows a range of costs based on sustained but low-intensity combat up to a force of 25,000 U.S. troops on the ground.

President Obama and the Pentagon have ruled out the the use of American boots on the ground, making the most expensive option the least likely. Yet as Todd Harrison, the lead author points out, war is "an unpredictable enterprise" and the ability to forecast its costs is limited.

Meanwhile, there are signs that the war may be shifting toward lower-intensity conflict. Already, pilots are finding fewer Islamic State buildings and infrastructure to destroy in Syria, hoping to pick off smaller enemy targets as they pop up, according to senior Defense Department officials. The Islamic State is also known as ISIL.

The move to combat patrols from mass attack to individual targets — a process called "dynamic targeting" by the military — is reflected in types of bombs and missiles fired. It also stems from the fact that, for all its bravado of claiming a caliphate over a broad tract of the Middle East, the Islamic State has few trappings of a traditional government — buildings, utilities or bridges, for instance.

The initial attacks last Monday focused on headquarters buildings, communication antennas and a terror training camp and barracks. Air Force planners, based on images provided by spy planes, identified targets in Syria. But a Defense official who was not authorized to speak publicly about the ongoing campaign said there are few fixed targets left for U.S. forces to strike.

An airstrike Friday could well be the template for the foreseeable future, the official said. Two U.S. F-15 fighters and two F-15s from Saudi Arabia were patrolling the sky over Syria when four tanks pilfered by Islamic State fighters were spotted. The jets promptly destroyed them.

Air Force pilots dropped 59 bombs in Syria with laser sensors that allow them to track and destroy vehicles traveling even at highway speeds, data from the Air Force show. The Air Force also fired 44 Hellfire missiles, which are often fired by Predator and Reaper drones. Both unmanned aircraft have been flying missions in Syria.

Over the weekend, U.S. and coalition forces fired on ISIL tanks, armored vehicles, checkpoints and safehouses, among other targets in Iraq and Syria, the U.S. Central Command said in a statement Sunday.

Harrison's estimate for this type of war, with about 2,000 U.S. troops on the ground in a support and advisory capacity, could cost up to $320 million per month, or $3.8 billion per year.

The early assessments show the strikes were successful, damaging or destroying the sites that were chosen, according to a senior officer who was not authorized to speak publicly.

While it may be more difficult, and airstrikes grow less frequent, significant damage has been inflicted the Defense Department official said. The presence of U.S. and allied warplanes means IS fighters will have more difficulty communicating, resupplying fighters in Iraq and moving on the battlefield.

Destroying the Islamic State, as Obama's strategy calls for, will require ground forces to re-capture territory and a legitimate government to hold it.

Ground forces will make the decisive difference against ISIL fighters, Army Gen. Martin Dempsey, chairman of the Joint Chiefs of Staff, told reporters Friday. Preferably not U.S. boots on the ground, he said.

"If you're suggesting that I might, at some point, recommend that we need a large ground force to counter ISIL, the answer to that is also absolutely," Dempsey said. "But it doesn't have to be Americans. In fact, ideally, for the kind of issues we're confronting there, the ideal force — in fact, the only truly effective force that will actually be able to reject ISIL from within its own population, is a force comprised of Iraqis and Kurds and moderate Syrian opposition."

House Speaker John Boehner, appearing on ABC's This Week Sunday, said that if no other nation can provide the needed troops for an on-the-ground force, he would support sending U.S. troops. "We have no choice," Boehner said. "These are barbarians. They intend to kill us. And if we don't destroy them first, we're gonna pay the price."

U.S. troops would also be expensive. A force of 25,000, backed by airpower, would cost as much as $1.8 billion per month, or as much as $22 billion per year, according to Harrison's estimate. The ground force accounts for 80% of those costs.

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