By MACKENZIE EAGLEN

On Thursday, the Pentagon will begin detailing its plans to cut $500 billion from the military’s budget over the next decade. The reason, insists President Barack Obama, is that “since 9/11, our defense budget grew at an extraordinary pace.” That’s true in top-line numbers—but it’s anything but true when examined strategically.

Between budget cuts, cost overruns, overweight bureaucracy, ever-growing red tape, and changing requirements, the arsenal of democracy has become a bureaucratic nightmare. In spite of itself, our military cannot build new programs anymore. Old programs might win wars, but with much higher human and financial costs.

After 9/11, defense budgets grew because they had to. The U.S. military’s budget, size and force structure had been too deeply cut in the 1990s, after the anticipated post-Soviet “peace dividend” failed to materialize. So the Pentagon began quickly and inefficiently dumping dollars into the military to fund the missions in Afghanistan and Iraq.

This made budgets grow steadily, but the money did little to increase cutting-edge capabilities for the future. Our war-related investments came at the expense of tomorrow’s military capabilities. As a new American Enterprise Institute study concludes, the military over the past decade didn’t modernize but rather embraced the equivalent of buying new apps for its old, clunky cellphone.

Associated PressThe Air Force wanted 750 F-22s to replace the F15, above, but they ultimately only received 187.

From 2000-2010, the Air Force spent $38 billion on 220 fighters—as compared to $68 billion for 2,063 fighters from 1981-1990. Air Force leaders wanted 750 F-22s to replace their F-15s, but successive administrations cut that number—to 648, then 438, 339, 270 and finally 187—before President Obama terminated production. That wasn’t a coherent acquisition strategy but budget-driven politics, plain and simple.

The Navy fared little better than the Air Force in terms of true modernization over the past decade. Sure, there are three Navy programs touted as “new”—the Virginia-class attack submarine, the DDG-51 destroyer and the F/A-18 Hornet. Problem is, those programs are already the Pentagon’s “Plan B.”

The Virginia-class sub was designed as a cheaper alternative to the truly dominant Seawolf-class attack submarine (and the Navy has bought less than half of the Virginia class, with the majority of funding still to come). The DDG-51 destroyer was a fall-back alternative to the now-canceled DDG-1000. And the F/A-18 Hornet and Super Hornets are only stopgap purchases until the Navy can put the F-35C Joint Strike Fighter on its carrier decks. Thus the Navy’s recent spending has gone to programs that are increasingly out of date and ill-prepared to meet the challenges of the 21st century.

The Marine Corps and Army fared better on modernization over the last decade. Though the Marine Corps saw its Expeditionary Fighting Vehicle canceled under President Obama, it successfully completed most of its ambitious amphibious docking platform.

The Army, meanwhile, completed several programs over the past decade, buying older platforms like the Abrams tank and the Bradley Fighting Vehicle, as well as the newer Stryker and even the Mine Resistant Ambush Protected (MRAP) armored vehicles. The problem is that all these vehicles are effective in land-based operations but would probably sit out during any conflict in the Western Pacific. And few of them have so-called next-generation capabilities—meaning brand-new platforms and technologies, not simply upgrades to existing tools. Almost every truly next-generation Army program has been canceled.

The common denominator among the services since 2001 is that their investment choices were geared toward lower-end conflict. Weapons systems designed for high-end future warfare suffered as a result (notwithstanding the evolving capabilities of drones over the past decade).

Compounding the problem is the reality that the services seem to be getting worse at acquiring high-tech systems and have used upgraded legacy programs as temporary band-aids. While it’s often important to get weapons out the door during a war, the unmistakable reality is that the momentum for innovative research and development seems nonexistent across the U.S. military.

What the Obama Pentagon will lay out this week is the final nail in the coffin of our national contract with our all-volunteer military—that if they fight, they’ll have the very best to win. It marks the beginning of the end of America’s unquestioned international military dominance. Our soldiers will increasingly go into combat with aged equipment, lacking assurance that they’ll prevail against any enemy.

Ms. Eaglen, a resident fellow at the American Enterprise Institute, has worked on defense issues at the Pentagon and on Capitol Hill.