WASHINGTON — When congressional cost-cutters meet later this year to decide on trimming the federal budget, the wars in Afghanistan and Iraq could represent juicy targets. But how much do the wars actually cost the U.S. taxpayer?

Nobody really knows.

Yes, Congress has allotted $1.3 trillion for war spending through fiscal year 2011 just to the Defense Department. There are long Pentagon spreadsheets that outline how much of that was spent on personnel, transportation, fuel and other costs. In a recent speech, President Barack Obama assigned the wars a $1 trillion price tag.

But all those numbers are incomplete. Besides what Congress appropriated, the Pentagon spent an additional unknown amount from its $5.2 trillion base budget over that same period. According to a recent Brown University study, the wars and their ripple effects have cost the United States $3.7 trillion, or more than $12,000 per American.

Lawmakers remain sharply divided over the wisdom of slashing the military budget, even with the United States winding down two long conflicts, but there's also a more fundamental problem: It's almost impossible to pin down just what the U.S. military spends on war.

To be sure, the costs are staggering.

According to Defense Department figures, by the end of April the wars in Iraq and Afghanistan — including everything from personnel and equipment to training Iraqi and Afghan security forces and deploying intelligence-gathering drones — had cost an average of $9.7 billion a month, with roughly two-thirds going to Afghanistan. That total is roughly the entire annual budget for the Environmental Protection Agency.

To compare, it would take the State Department — with its annual budget of $27.4 billion — more than four months to spend that amount. NASA could have launched its final shuttle mission in July, which cost $1.5 billion, six times for what the Pentagon is allotted to spend each month in those two wars.

What about Medicare Part D, President George W. Bush's 2003 expansion of prescription drug benefits for seniors, which cost a Congressional Budget Office-estimated $385 billion over 10 years? The Pentagon spends that in Iraq and Afghanistan in about 40 months.

Because of the complex and often ambiguous Pentagon budgeting process, it's nearly impossible to get an accurate breakdown of every operating cost. Some funding comes out of the base budget; other money comes from supplemental appropriations.

But the estimates can be eye-popping, especially considering the logistical challenges to getting even the most basic equipment and comforts to troops in extremely forbidding terrain.

In Afghanistan, for example, the U.S. military spent $1.5 billion to purchase 329.8 million gallons of fuel for vehicles, aircraft and generators from October 2010 to May 2011. That's a not-unheard-of $4.55 per gallon, but it doesn't include the cost of getting the fuel to combat zones and the human cost of transporting it through hostile areas, which can hike the cost to hundreds of dollars a gallon.

Just getting air-conditioning to troops in Afghanistan, including transport and maintenance, costs $20 billion per year, retired Brig. Gen. Steve Anderson told National Public Radio recently. That's half the amount that the federal government has spent on Amtrak over 40 years.

War spending falls behind tax cuts and prescription drug benefits for seniors as contributors to the $14.3 trillion federal debt. The Pentagon's base budget has grown every year for the past 14 years, marking the longest sustained growth period in U.S. history, but it seems clear that that era is ending.

Since the U.S. government issued war bonds to help finance World War II, Washington has asked taxpayers to shoulder less and less of a burden in times of conflict. In the early 1950s Congress raised taxes by 4 percent of the gross domestic product to pay for the Korean War; in 1968, during the Vietnam War, a tax was imposed to raise revenue by about 1 percent of GDP.

No such mechanism was imposed for Iraq or Afghanistan, and in the early years of the wars Congress didn't even demand a true accounting of war spending, giving the military whatever it needed. Now, at a time of fiscal woes and with the American public weary of the wars, the question has become how much the nation's largest bureaucracy should cut.

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"The debt crisis has been a game changer in terms of defense spending," said Laura Peterson, a national security analyst at Taxpayers for Common Sense, a nonpartisan budget watchdog.

"It used to be that asking how much the wars cost was unpatriotic. The attitude going into the war is you spend whatever you cost. Now maybe asking is more patriotic."

Still, deep cuts to the Pentagon remain unpalatable to many lawmakers. The debt limit deal that Congress passed earlier this month calls for $350 billion in "defense and security" spending cuts through 2024, but that's expected to be spread across several government agencies, sparing the Pentagon much of the blow.

However, if the 12-member bipartisan "super-committee" of lawmakers can't agree on further federal budget cuts later this year, the law mandates across-the-board cuts of $1.2 trillion over 10 years, with half of that coming from the Pentagon. The prospect of such deep defense cuts is thought to provide a strong incentive for deficit hawks to compromise and spread the pain more broadly.

Politics aside, finding defense savings is complex, even with the Obama administration trying to wind down two wars. For one thing, reducing troop levels doesn't necessarily yield commensurate cost reductions, given the huge amount of infrastructure the military still maintains in each country.

In Afghanistan, the cost per service member climbed from $507,000 in fiscal year 2009 to $667,000 the following year, according to the Congressional Research Service. Fiscal year 2011 costs are expected to reach $694,000 per service member, even as the U.S. military begins drawing down 33,000 of the 99,000 troops there.

In Iraq, even with the overall costs of the war declining and the U.S. military scheduled to withdraw its remaining 46,000 troops by the end of this year, the cost per service member spiked from $510,000 in 2007 to $802,000 this year.

In fiscal year 2011, Congress authorized $113 billion for the war in Afghanistan and $46 billion for Iraq. The Pentagon's 2012 budget request is lower: $107 billion for Afghanistan and $11 billion for Iraq.

In the more austere fiscal climate, the Pentagon has tried to be proactive, proposing cuts to some major military programs such as the controversial and hugely expensive F-35 Joint Strike Fighter.

Adm. Mike Mullen, the former chairman of the Joint Chiefs of Staff, has called the national debt the biggest threat to U.S. national security. Before leaving office last month as defense secretary, Robert Gates ordered his department to find ways to cut $400 billion from the defense budget over 12 years, under Obama's orders.

Among the challenges of determining the costs of war is defining what to include. Rising health care costs for veterans? The damage done to Iraqi and Afghan families, cities and institutions? Holding tens of thousands of detainees at U.S. military prisons in those two countries and others around the world? The massive interest on war-related debt, which some experts say could reach $1 trillion by 2020?

"The ripple effects on the U.S. economy have also been significant, including job loss and interest rate increases, and those effects have been underappreciated," wrote a team of Brown University experts who authored a June report called "Costs of War."

Critics of the defense budget process note that the U.S. already has paid a heavy cost for the wars, spending billions to wind up with older equipment and troops receiving less training.

Winslow Wheeler, who worked on national security issues on Capitol Hill for 30 years, said the Navy and Air Force fleets were smaller after a decade of war. The Army has been left with run-down, overworked vehicles and equipment.

"The danger of that is that as we blithely go on not paying attention, things happen that we don't notice, like the older, less trained forces," Wheeler said. Because the cost of replacing equipment has risen dramatically over the past decade, "what we are paying is a higher cost for a smaller force." He likened it to replacing a Lamborghini with a Volkswagen.