The United States in August and September began considering in earnest whether or not to become militarily involved in Syria. There are many tough and contentious questions about that decision, but one fact is undeniable: It would be expensive.

In a 2010 paper, Stephen Daggett of the nonpartisan Congressional Research Service estimated the costs of all major U.S. wars expressed in contemporary dollars, from the American Revolution through the current conflicts in Iraq and Afghanistan. With the caveat that comparing war costs over a 230-year period is "inherently problematic" because the definition of war has varied and official numbers have included and measured different things over time, and also "because of the difficulties in comparing prices from one vastly different era to another," Daggett nonetheless concludes that the trend is clear: Wars aren't cheap.

According to his estimates, the American Revolution cost $2.4 billion (all numbers are in constant FY2011 dollars), World War I cost $334 billion, World War II cost $4.1 trillion, and the Iraq and Afghanistan wars combined have cost around $1.1 trillion and growing.

The price tag on the proposed intervention in Syria is unclear. According to a Congressional Research Service report published in September, "the cost of any military intervention could range widely depending on the type and length of U.S. military actions, the participation of U.S. allies, and Syrian and Syrian-allied responses." Estimates range from $500 million initially to train, advise, and assist opposition forces in a safe area outside Syria, to as much as $12 billion dollars a year to use military force to establish either a no-fly zone that would prevent the regime from using its aircraft or a buffer zone to protect border areas next to Turkey or Jordan.

If history is any guide we can expect that direct military spending will be grossly underestimated. Take the 2003 war in Iraq. Mitch Daniels, then the director of the Office of Management and Budget, predicted that the war in Iraq would cost $50 to $60 billion, including the costs of reconstruction and clean-up. President Bush's economic advisor Larry Lindsey got canned for suggesting that it could cost as much as $100 billion. But as of 2013, the Cost of War Project estimates that Department of Defense (DOD) appropriations for the Iraq and Afghanistan wars combined has already reached $1.4 trillion. On top of DOD appropriations, the total costs of the Iraq war alone-including war-related expenses through 2013-is about $1.7 trillion. This number increases to $2.1 trillion when you add substantial costs for veterans through 2053.

There are several reasons for the discrepancy between government-projected and actual costs of war. The first one is mission creep. We tend to intervene more deeply than originally planned, from Vietnam to Afghanistan. Second, pro-war interests have an incentive to make the war appear to be as cheap-and therefore attractive-as possible. Secretary of State John Kerry claimed during a September 4 hearing on President Obama's request for an authorization for use of military force in Syria that unnamed "Arab countries" have offered to pay for the administration's proposed war in Syria. Something similar happened in March 2003 when the Bush administration announced that the war in Iraq would be paid for by future oil revenues there.

But no matter how large the direct price tag for war, that's only a down payment on what military interventions usually end up costing taxpayers. Queens College economics professor Ryan D. Edwards looked at the costs of 10 major U.S. wars in a 2010 National Bureau of Economic Research paper, and found that the costs of death and disability compensation, health care, and survivors' benefits to veterans and their dependents "represent between one third and one half of the total present value of all war costs."

Even for short and seemingly cheap engagements, like the 2011 Libya intervention and possibly Syria this year, the unfunded liability of future veterans' benefits will have lasting implications for fiscal policy beyond the short-term impact on the budget and economy. That was certainly the case for both the Spanish-American War and the first Gulf War.

Any Syria-war costs would also be higher than originally announced because lawmakers will abuse the "emergency" spending process. During the two-front war in Iraq and Afghanistan, this back-door budgeting scheme allowed President Bush and his Republican Congress to not only conceal the true costs to taxpayers, but also to avoid painful budget choices while funneling billions of dollars in unvetted goodies to favored military contractors. The result was more defense and nondefense spending. Lawmakers from both parties have refused to close this emergency loophole, even though Obama has so far kept his promise to fund the war out of regular appropriations.

There's another factor that could trigger higher-than-projected costs in Syria. As soon as news of a potential U.S. strike hit the news cycle, hawks and pro-defense spending interests started making the case that it would require boosting defense spending or putting an end to the defense spending cuts implemented on March 1 of this year though sequestration. During a September 8 interview on CNN's State of the Union program, for instance, Rep. Howard P. "Buck" McKeon (R-Calif.), chairman of the House Armed Services Committee, warned that many Republicans would only support the president's intervention if it was coupled with repeal of the modest defense cuts.

The cost implications of undoing hard-fought sequestration could be large. Democrats wouldn't agree to re-boosting defense spending without undoing the rest of the sequestration cuts as well. If that happens, as with emergency supplementals, both defense and nondefense spending will grow much faster than projected.

For all these reasons, Americans should demand that lawmakers do a much better job at projecting all the costs of an intervention in Syria. Then, they need to demand a credible plan on how it will be paid for. Under no circumstances should war ever again simply be added to the nation's credit card.