On November 2, Major Stephen Stilwell of the Missouri Air National Guard was taking his F-15 Eagle through its paces when the plane did something for which it hadn't been engineered: It cracked into pieces.

Maj. Stilwell survived the accident, but the F-15 fleet--America's signature fighter for 30 years--may not. This isn't just some maintenance issue, but goes directly to the question of whether the United States intends to deploy the world's best Air Force or one that (fingers crossed) is good enough.

The Air Force has since discovered significant stress fractures in at least eight other aircraft, and ordered that 442 of the older-model F-15s be grounded through at least January (though 224 of the newer-model F-15Es continue to fly). Those 442 Eagles, or about a fifth of the total number of fighters fielded by the Air Force, are mainly responsible for homeland defense. They're the ones that would have to be scrambled to intercept hijacked jetliners in the event of another 9/11.

In an alternative universe, the F-15 problem would not be significant, because the Air Force would already be flying large numbers of its designated replacement, the F-22 Raptor. But the Raptor--a fifth-generation fighter that outclasses everything else in the sky--was deemed too costly and too much of a "relic" of the Cold War. The Air Force currently has orders for no more than 183 of the planes (with some Raptor squadrons already fully operational), though there is now talk of keeping the production line open for as many as 200 more. We think it's an investment worth making.

Before the F-15's problems became so glaring, it was plausible to argue that the plane was adequate to meet current defense needs until the F-35 Joint Strike Fighter--still in its testing phase--comes into service sometime in the next decade. But while the Air Force will surely engineer whatever patch the grounded Eagles need to make them airworthy again, it cannot patch the fact that it may be six months or longer before the fleet is back to full operational readiness. This is hardly trivial for a force already strained by wars in Afghanistan and Iraq and threats that stretch from the Korean Peninsula to the Horn of Africa.

Nor is there any getting over the fact that the F-15 first flew in 1972--long before many of the current crop of pilots were born--and that the plane is now outclassed by its competitors in the export market. In 2005, a British Eurofighter reportedly defeated two F-15Es in a mock dogfight. Simulated dogfights have also shown that the F-15s are somewhat inferior to Russia's more modern Su-35s.

Some defense experts claim the era of air-to-air combat is over, but similar erroneous forecasts have been made before. It's also far from clear that the single-engine F-35 can be considered a genuine replacement for the twin-engine F-15 or an adequate substitute for the (also twin-engine) F-22. The F-35 is something of a hybrid plane, with at least one version of it having a Harrier jet's vertical take-off and landing capabilities, and is also destined for shipborne service. Its great virtue is that it's a cheaper plane, but its performance is in many ways compromised by the various roles it's meant to play. As a fighter, it cannot compete with the Raptor.

As for cost, there's no doubt that at more than $100 million per additional plane, the Raptor is an expensive aircraft. But estimates of the plane's price tag typically factor in research and development costs, meaning the price per plane actually increases the fewer we build. And with a defense budget at roughly 4% of GDP (compared with a mid-1980s' peak of more than 6%), we have a long way to go before any weapons system is more than the U.S. can really afford.

The issue, then, is whether the U.S needs the best plane in the sky. For all the talk of the F-22 being a legacy of the Cold War, we are far from convinced that the U.S. will forevermore be faced with only Taliban-like adversaries incapable of fielding air forces of their own, or that the era of great power military rivalries is over. Judging by the expensive weapons systems currently being developed in China and Russia (which on Tuesday successfully tested a new ICBM, apparently Vladimir Putin's idea of the Christmas spirit), it seems that neither country has reached that conclusion either.

We cannot predict what kind of adversaries the U.S. will face in the coming decades, but we do know that part of the responsibility of being the world's "sole remaining superpower" is to be prepared for as many contingencies as possible. One prudent way of reducing the threat is to discourage potential adversaries from trying to match America's advantages in numbers and technology. Replacing our faltering Eagles with additional Raptors may be expensive, but allowing our neglect to be exploited by those who wish us harm would be ruinous.