Serious talk of America's defense budget was largely absent from the final weeks of the presidential campaign, once President Barack Obama likened Gov. Mitt Romney's concerns to an anachronistic focus on "horses and bayonets." But when Mr. Romney lamented that (among other things) the U.S. Air Force has the fewest airplanes it has ever had, he was correct. At its founding in 1947, it had more than 12,300 planes. Today: approximately 5,200.

As the Air Force has been retiring large numbers of older aircraft in recent years, its budgets—drafted by the Pentagon and ultimately enacted by Congress—have prevented it from acquiring enough new aircraft to perform the missions of those retired. From 2008 through 2012, the Air Force retired 700 more aircraft than it bought.

Nevertheless, the Obama administration's budget request for fiscal year 2013 sought to retire an additional 300 airplanes while buying only 54 new ones—a proposal that Congress has so far refused to endorse. The last time the U.S. bought so few aircraft was 1915 (for the Aviation Section of the U.S. Army Signal Corps, an Air Force predecessor). The U.S. even bought more aircraft during the Great Depression.

Then there is the matter of how America's offensive and defensive capabilities have weakened over the past 20 years compared with those of its adversaries and potential adversaries. From that perspective, Mr. Romney's critical statements on defense cutbacks didn't begin to portray how perilous is the state of the Air Force—and how soon its weaknesses might begin endangering missions essential to U.S. national security.

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Associated Press

An Air Force ground-crew member works on a KC-135 Stratotanker at Kadena Air Base, Okinawa, Japan, on Aug. 14. The airplane was built in 1958.

Russian and Chinese aircraft, flown by Indian pilots in exercises, have already bested the U.S. Air Force's fourth-generation aircraft, F-15s and F-16s. Both Russia and China have developed fifth-generation fighters similar to the Air Force's F-22 and F-35.

Moscow and Beijing say they intend to produce those aircraft in numbers far greater than does the U.S., and they have announced plans to sell them to other countries. Russia and China are also creating air defenses that will challenge all but the most sophisticated U.S. aircraft, as are Iran, Venezuela and other countries.

Also troubling are the recent closures of several Air Force production lines. This means that if current estimates of Air Force needs turn out to be too low—for example, if the rising capabilities and intentions of Russia or China become greater threats than anticipated—then the U.S. won't be able to act quickly in response. The production lines and the people who man them will be gone.

Over the past four years, the Obama administration terminated or delayed seven aircraft production lines, including those of the F-22 fighter jet, the C-17 transport airplane, the replacement search-and-rescue helicopter, and a new bomber.

These cuts affect not only Air Force capabilities but also the aerospace industry, which currently accounts for 10 million jobs and almost $50 billion in net exports—the largest of any American industry, according to the Aerospace Industries Association. Closing production lines has meant losing thousands of defense jobs over the past three years. With $500 billion in "sequestration" cuts scheduled to begin on Jan. 2—as part of $1.2 trillion in automatic cuts agreed to in Washington after budget talks failed in 2011—the defense industry will lose another 2.13 million direct and indirect jobs, according to a study from George Mason University.

Building an air force is a long-term process. The reason that the military branch in the U.S. has performed so well is that the American people nourished it with decades of sustained and consistent investment. Yet the Air Force's fleet of planes is older than it has ever been—more than 25 years on average. Some planes, such as the B-52 and KC-135, are more than 50 years old. Americans probably wouldn't let their children or grandchildren take a car that old out on the interstate, or operate it when the temperature is 40 degrees below zero, especially if the vehicle already had hundreds of thousands of miles on it and needed repair.

Ironically, the inattention and repeated cuts that have taken a toll on this branch of the military haven't received the public attention they deserve because the Air Force has been so successful. No U.S. soldier has been killed by enemy air power since 1953. For six decades the Air Force has been able to deny operational air space to adversaries, so U.S. ground forces have operated with little fear of enemy aircraft attacking their positions.

But the U.S. relies on the Air Force to do much more than that—including to hold at risk any actual or potential enemy target, anywhere in the world; to protect the ground forces of friends and allies, such as the Northern Alliance in Afghanistan or the freedom fighters in Libya; to protect the U.S. from a nuclear attack; to reach into Iran if called upon; to provide navigation through its global positioning systems; to gather intelligence remotely from far-flung areas; to defend against cyberattacks; to airlift humanitarian aid anywhere in the world; and otherwise to deter potential adversaries.

What the Air Force can't do is continue on its present path. At least not for much longer