Andy Marshall is an evil genius. During the Nixon administration, he set up shop in the White House to help rethink how to deal with the Soviet menace.

In 1973, he shifted to the Pentagon. There he pioneered "net assessment," a new way of thinking about military force.

Measuring military power entailed more than counting how many ships, planes, tanks and nukes each side had, Marshall argued. One had to consider all factors — from the state of training and maintenance to the political cohesion among allies — to determine the relative power of East and West.

Marshall's way of looking at war influenced U.S. strategic thinking throughout the Cold War. And he and his Office of Net Assessment are still at it, advising senior government officials on how to think about future challenges to American security.

But Marshall must be flummoxed by President Obama's approach to strategic planning. Every time the president “rethinks” what America needs to assure its security, the answer turns out to be “less.”

Defense cuts have been the president's only real contribution to reining in spending. They have reduced strategic planning to simple math: How much can the Pentagon do, given the money it will receive?

Obama argues that is what military forces must do in an age of austerity. But that’s like a hospital accountant saying surgeons just have to do the best they can without surgical equipment: “As long as the numbers add up, everything is fine.”

Except the mission of the hospital is to save lives. The patients who will die are not as sanguine as the accountant.

Strategy is more than arithmetic. In “Strategic Thinking in 3D,” Georgetown professor Ross Harrison makes the case that it's wrong to think of strategy as simply an exercise in resource allocation.

Sound strategy must be centered on "capabilities,” he argues, and “manufacturing” capability involves more than merely identifying and allocating resources.

By any measure, U.S. capabilities are now atrophying, even as the strategic challenges they are supposed to deal with are strengthening.

China is a case in point. The U.S.-China Commission will soon issue a report on the trajectory of Beijing’s maritime power that will curl the hair of strategic planners at the Pentagon.

Meanwhile, Obama is on course to leave America with a smaller navy in 2017 than when he started in 2009. No wonder Obama’s “pivot to Asia” is but a punchline in Beijing.

Missile defense is another example. That budget has been gutted. Last year, the administration admitted it hadn't planned for enough missile defense to keep up with the North Korean threat — much less stay ahead of our adversaries.

Meanwhile, the Pentagon is busy jettisoning ground troops, divesting the nation of the most experienced, capable military forces in our history, so it can pay its bills.

It’s like selling your SUV, knowing that when you need the capability to go cross-country in the future, all you'll be able to field is a tricycle.

As Harrison warns in his book, "it is easy to lose sight of capability if one has his or her eye mostly on resources." Obama has committed that strategic error many times over.

With Washington twisted in political knots over the current budget battle, now may seem to be the wrong time to worry about military capability. It isn’t. The president and the Congress have a constitutional duty to "provide for the common defense."

Duty comes first. It calls on them to stop turning limited government into big government and, instead, make the fiscally responsible decisions necessary to provide the military capabilities America needs.

 - James Jay Carafano, a Washington Examiner columnist, is vice president for defense and foreign policy studies at the Heritage Foundation.