George W. Bush was right before he was wrong. Mr. Bush initially opposed the creation of the Department of Homeland Security, but he bowed to political pressure and formed a new bureaucracy and increased domestic-security funds. Ten years after Sept. 11, it’s time to rethink the very existence of that department because the additional layers of government and wasteful spending do not provide enough security to justify its existence.

In response to the terrorist attacks, Mr. Bush created a White House advisory position, with Tom Ridge coordinating federal terrorism prevention and response. Members of Congress thought that plan was insufficient and called for a sweeping reorganization of government. Mr. Bush suddenly embraced the idea and pushed for a Cabinet department larger than anything Congress proposed. What’s more, Mr. Bush claimed the new department would be revenue-neutral, a claim that is laughable in retrospect.

In creating Homeland Security, Congress lumped together 22 previously unconnected federal agencies under a new Cabinet secretary. That’s a problem, not a solution. And while members of Congress routinely clamor for consolidating Homeland Security oversight in one committee, that seems unlikely: 108 congressional committees and subcommittees oversee the department’s operations. If aggregating disparate fields of government made any sense in the first place, we long ago would have consolidated all Cabinet responsibilities under one person — the secretary of government.

Rather than rethinking the structure that imposes inefficiency in the first place, Congress is building Homeland Security a new headquarters to help manage the mess it created. Unfortunately, the largest Washington-area federal construction project since the Pentagon will hold just 14,000 of the 35,000 staff members in the national capital region — less than half. The department projects that spending $3.4 billion to build this massive facility will save $400 million in management costs over the next 30 years. The math is dubious, primarily because few costs can be projected over 30 years with any certitude. And only in Washington would spending several billion dollars to save hundreds of millions be considered a bargain.

Shockingly, the cost of the new headquarters roughly equals what we’ve averaged annually on homeland security grant programs to cities and states. That’s part of the political appeal of “homeland security.” It allows politicians to wrap pork in red, white and blue in a way not possible with defense spending. Not every town can host a military installation or build warships, but every town has a police force that can use counterterrorism funds to combat gangs or a fire department that needs recruits or a new fire station.

You’d think that after Sept. 11 we would have focused our spending on securing the obvious terrorism targets, and in some cases we have. Too often, however, we’re not doing that — like when we buy a hazardous-materials trailer for a county in rural Ohio, a piece of equipment that later gets sold because it sits unused in a parking lot and costs too much to maintain. Or when we buy an 80-camera surveillance system to monitor an Alaskan town too small for a streetlight. Or when we satiate officials’ desire for NASA-style command centers and buy 55 flat-screen televisions for a local intelligence center only to find out that the TVs are all tuned to the same news channel.

There’s reason to doubt how much security this spending has bought us. A study by professors John Mueller and Mark Stewart found that in order to survive a cost-benefit analysis, the past decade’s increased homeland-security expenditures “would have to deter, prevent, foil or protect against 1,667 otherwise successful [attempted Times Square car bomb]-type attacks per year, or more than four per day.”

Ultimately, it may be fiscal necessity that reduces our bureaucratic bloat. The decade since Sept. 11 has been one of profligate overspending, and now the long-feared entitlement crisis is upon us. Government needs to get leaner and spend security dollars on those programs that are cost-effective, scrapping or downsizing the ones that are not. While many of the agencies under the Department of Homeland Security have valid federal missions and would not go away, such as the Secret Service and Border Patrol, we are due for a serious belt-tightening. Abolishing the department, an agency born of political and economic overreaction, would be a good start.