This simple equation is nothing new. Three years ago, the Senate Budget Committee adopted a bipartisan amendment requiring that wars be paid for. The Simpson-Bowles deficit-reduction commission and Senator Al Franken, Democrat of Minnesota, both proposed doing much the same thing. None of these proposals resolved the question of whether to pay for future wars through spending cuts or raising more revenue. Now that Congress has finally passed legislation letting taxes increase, we must make a choice and require a tax surcharge to pay for any military operation.

War traditionally has motivated major changes in tax policy. The Civil War brought the first income tax. World War I made the federal income tax permanent. World War II brought tax withholding. In 1969, at the height of the Vietnam War, the United States ran a budget surplus because of a tax surcharge Congress forced President Lyndon B. Johnson to accept.

Today’s budget negotiations offer a similar opportunity to make a surcharge permanent. President Obama called for counting as savings the money that will not be spent as the war in Afghanistan winds down. Many decried the scheme as playing with funny money because he plans to exit Afghanistan in 2014 anyway; the savings only exist because of an accounting trick in Congressional budgeting. But if those savings were associated with an actual policy change, they would start looking more real.

Since the Budget Control Act already caps military spending, there is an easy way to implement the surcharge: any spending over the caps would require it. If we felt the need to use the military and could do so under the spending caps, as the Obama administration did in 2011 responding to the earthquake in Japan and the uprising in Libya, no surcharge would be necessary. But if military action required supplemental financing, any amount over the caps would be offset with new revenue raised by an automatic surcharge on taxes.

By tying military action to additional revenue, the president would actually have a freer hand in deciding when to use force. Every argument the Obama administration makes for military action would explicitly include a call for increased taxes, forcing the question of whether the stakes in the military situation are worth the cost. If the American people agree they are worth it, the president will get both the political support and financing he needs.