Fiscal responsibility and national security are not mutually exclusive. No budget should escape scrutiny in these tough economic times. But in calling for the Pentagon to kick in an additional $400 billion through spending cuts, President Barack Obama has put the budgetary cart before the horse.

To cut defense responsibly, the president must first decide which of our foreign commitments can be abandoned safely. This is no easy task, given the scope of threats we face. And the president has not done this. Instead, he has directed defense officials to conduct yet another review to identify $400 billion of expenses they can jettison.

This is precisely the wrong approach for defense budgeting. The purpose of defense is to safeguard our people, our interests and our sovereignty and deter potential threats. Civilian and military leaders are responsible for identifying salient threats and the resources needed to assure we can overcome them. Then, and only then, should they craft a budget — one that supports the identified essential missions and needs.

A directive to come up with $400 billion in defense spending cuts presumes that the U.S. can and will take a less-active role in world affairs. Yet no president of either political party has backed away from America’s global leadership role, much less significantly reduced our nation’s commitments by treaty or interest.

Indeed, time after time, America has found itself with no choice but to sally forth, no matter how reluctantly. Bill Clinton would have much preferred a subordinate role in the Bosnian conflict. But that proved impossible when genocide in the Balkans threatened the stability of Europe and America’s relationship with the Muslim world.

Obama is but the latest in a series of presidents to increase the number and scope of U.S. military missions over the past 15 years. In little more than a year, he has surged forces in Afghanistan, amped up counterterrorism operations with Pakistan and Yemen, launched a new military operation in Libya and sent troops to aid disaster victims in Japan and Haiti. As Colin Powell recently noted, the military is being asked to do more than the troops in World War II did — and not just in Afghanistan.

Yet soon they will be asked to do more with far less. As the $400 billion ax falls, the first casualties will be next-generation weapons programs. This is where Defense Secretary Robert Gates has so far focused most of his budget cutting.

Last year, Congress and the administration cut $300 billion by canceling programs that included the F-22 fifth generation fighter, the Army’s future combat systems (primarily a ground vehicle program), the multiple-kill vehicle for missile defense, an Air Force bomber, the VH-71 presidential helicopter, a transformational satellite program and the second airborne laser aircraft. The administration also extended construction of an aircraft carrier by an extra year and delayed indefinitely the Navy’s next-generation cruiser.

It’s more of the same in this year’s defense budget and what the president proposes for next year. Planned reductions include permanently canceling the cruiser; ending production of the C-17 (our only wide-bodied cargo aircraft); terminating the Navy’s EPX intelligence aircraft; ending another satellite program and killing the expeditionary fighting vehicle program for the Marine Corps.

Also on the block is the Army’s surface-to-air missile program and its non-line-of-sight cannon.

The latest deficit reduction effort will wipe out more modernization programs. At risk are the Army’s ground combat vehicle, the Air Force, Navy and Marine Corps’s Joint Strike Fighter, the Navy’s DDG-1000 next-generation destroyer, the Air Force’s trainer, the Navy’s Littoral Combat Ship, the Army and Navy Joint High Speed vessel, the Air Force’s new combat search and rescue helicopter, the Army and Air Force Joint Cargo Aircraft and various space, satellite, and missile defense capabilities.

More programs planned or in early implementation stages also would be killed or deferred indefinitely. In this category are the Air Force’s new bomber, the Navy’s next-generation nuclear submarine, the Marine Corps’s amphibious combat vehicle and a next-generation Army

“Legacy” weapons systems also would take a hit. Expect the Navy to buy fewer destroyers, carriers, submarines, littoral combat ships and aircraft than planned and to lengthen time between maintenance on ships. The Army and Marine Corps may not upgrade as many Humvees as expected. The Air Force may reduce the buy of next-generation aircraft and try to extend the service life of its existing aircraft.

Why raid the equipment accounts for savings? New platforms always are the first to go, even though they account for just one-seventh of the military’s budget, because everything else — from personnel to current operation costs — are considered “must pay.” Historically, when defense budgets come down, modernization programs fall first and fastest.

The problem, of course, is that the sheer magnitude of the additional spending cuts being proposed would undercut everything from end strength (the number of people in uniform), to readiness and training, to base facilities and infrastructure.

Much of the equipment our military flies, sails and drives already is 20 to 60 years old, and it’s wearing out at wartime rates. The cuts being proposed today make even full replacement dubious. Modernizing to counter future threats would be impossible under the predetermined bottom line.

Yet those who demand scaling back military size, structure and capabilities in the name of fiscal prudence ignore the fact that we’ll have to spend more later to rebuild.

It sounds quite efficient and tough-minded to assert that our military will simply do more with less. But the fact is, any attempt to do more with less inevitably will create vulnerabilities in a defense stretched too thin. And any resolve to simply do less will go flying out the window when the next humanitarian crisis arises or when our allies ask for help in an emergency.

Starving the military to meet purely financial goals makes the country less safe. It increases our vulnerability and invites our enemies to probe for — and exploit — our weaknesses, boosting the likelihood of our being drawn into yet another overseas mission. It’s a penny-wise, pound-foolish approach to deficit reduction.

Mackenzie Eaglen is a research fellow for national security studies at The Heritage Foundation’s Douglas and Sarah Allison Center for Foreign Policy Studies.