THE COST of the new U.S. military operation in Iraq and Syria is already approaching $1 billion, according to a study released last week. The Pentagon has meanwhile launched a $750 million mission to fight Ebola in Africa and has committed to rotating U.S. troops through NATO countries bordering Russia. These are all justified initiatives with broad support from Congress and the public. But the budgetary foundation needed to sustain them is crumbling.

President Obama’s abrupt reversal of his policy of winding down U.S. foreign military engagements follows years of sweeping cuts in the defense budget. Since 2010, defense spending has been slashed 21 percent in real terms, according to the Center for Strategic and Budgetary Assessments , which also produced last week’s war cost study. The Army has announced plans to reduce its size to the lowest level since before World War II, while the Navy is on its way to the smallest number of ships in a century. More huge cuts are mandated under the fiscal plan passed by Congress in 2011, which would reduce Pentagon spending over a decade by $1 trillion from that proposed by Robert Gates when he was defense secretary.

In the short term, the new war against the Islamic State and the Ebola mission in West Africa will be covered out of the fund Congress appropriates every year to cover “overseas contingency operations.” But even that budget has come under pressure: The Defense Department’s original request of $58.6 billion for the fiscal year beginning Oct. 1 was down by a quarter from the year just ended.

The undeniable reality, as a bipartisan commission created by Congress recently concluded, is that the 2011 budget plan was a “serious strategic misstep” that has hamstrung the Pentagon’s ability to respond to the multiple international crises of this year. The problem will surely worsen as the war against the Islamic State — which Mr. Obama and military leaders have said is likely to last for years — deepens. Last week’s study estimated that “lower---intensity air operations” in Iraq and Syria, along with 2,000 personnel on the ground, could cost $2.4 billion to $3.8 billion over a year, and the price could soar past $20 billion if the fighting intensifies and a larger ground contingent is needed.

Some of the war costs could be covered if Congress were to accept the simple, common-sense changes in military health-care spending and retirement costs that the Obama administration has repeatedly proposed, along with base closures and cuts in the civilian and contractor workforce. But even an unlikely end to the bipartisan political opportunism that prevents such reforms would leave the Pentagon underfunded by hundreds of billions of dollars in the coming years.

Congress should follow the recommendation of the National Defense Panel and restore the defense budget to the levels Mr. Gates recommended in 2011. Then it should provide the revenue to cover those costs and those of the new war. A substantial part of the fiscal trouble the country faces was caused by Congress’s choice to authorize more than a trillion dollars in war spending in Iraq and Afghanistan without raising revenue or making compensatory budget cuts. That mistake should not be repeated.