There is something unnerving about watching Iranians browse weapons built to hunt down and destroy American warplanes.

The International Salon of Weapons and Military Equipment—2010, held at the famed Zhukovsky airfield outside Moscow, outwardly resembles U.S. defense industry shows. Exhibits stand in rows inside a cavernous hangar converted into a convention hall. Engineers and sales flacks talk up their wares. Employees hand out pens tattooed with company names and logos. Clusters of visitors—on the first day of the show, mostly potential international customers—gather at the displays. Here, a couple of Eastern Europeans peer through the scopes of unloaded sniper rifles. There, a group of Asians gawk at a demo of small radio-controlled quadrotors.

This is all pretty standard defense industry fare. But some differences become more apparent when I reach the booth of the Russian firm Almaz-Antey, one of the world's leaders in antiaircraft weaponry and the nation's largest arms dealer. A promotional animation on a large screen hanging over the display shows an Almaz missile streaking toward an airplane that looks a lot like a carrier-launched F-35C Lightning II. The missile closes and the airplane disappears in an orange explosion.

The image is shocking—I'm used to seeing American stealth warplanes prevail, in combat as well as in corporate promotional animations. The U.S. government has invested 16 years and $396 billion to ensure that F-35s can fly undetected through well-defended airspace. And the Russians are selling defense systems that can knock them out of the sky?

I notice a trio of men in nearly identical gray suits and close-cropped beards examining toy-train-size models of mobile radar and missile launchers. They are from the Sharif University of Technology in Tehran, a civilian institution that has ties to the military. Late last year, a scientist from Sharif visiting the United States was arrested for purchasing unspecified equipment that could be used in military programs.

Almaz engineer Ivan Shalaev sidles next to them and they settle into a conversation in English. It's a perfect opportunity to eavesdrop. The Iranians ask Shalaev questions about infrared sensors that can detect an airplane by the heat of its engines and the air friction against its skin. But Shalaev tells the Iranians that infrared is just one tracking method the company offers to customers.

Behind him are seekers that use enhanced radar to chase down warplanes. Several are cut open to show a gimballed disc studded with a forest of tiny T-shaped transmit/receive modules. Under the disc is a small computer that can quickly process even the most subtle radar returns. This makes the missile responsive and difficult to outwit. Almaz-Antey is selling these upgraded warheads to fit on existing antiaircraft missiles, including ones it sold to Syria, Venezuela, China, and Iran.

The Iranians don't answer any direct questions, beyond stating their university affiliation, when I introduce myself as an American journalist. But Shalaev is open, even friendly. He's a hometown boy; his father was an engineer, too, who worked on advanced Russian aerospace programs here at the Zhukovsky.

The young engineer is not shy about which airplanes are in his company's crosshairs. Asked if the new seekers could track and destroy an F-35, Shalaev grins and says, "Well, we're going to try."

Two years after the Zhukovsky Arms show, sales of Russian antiaircraft equipment are surging, and Almaz-Antey is at the head of the effort. Company officials, quoted in Russian media, say that the nation's new defense plants—the first built in 20 years—will make antiaircraft weapons.

That's not good news for U.S. pilots and American allies. The Pentagon strives to hold any place in the world, no matter how well defended, under threat of air attack. Modern U.S. warplanes are designed to evade enemy radar, electromagnetic snoopers, and heat-seeking missiles. The Pentagon calls this low observable (LO); the rest of the world calls it stealth.

LO aircraft enable precision strikes against protected targets, allow military containment of rogue states, and curb the geopolitical appetites of potential adversaries. Their existence creates diplomatic leverage. Selling antiaircraft weapons to anyone who feels threatened by the U.S. and NATO is a way of making Russia a global power player.

"Russia still believes it has an important role to play in the world," says Travis Sharp, an analyst with the Center for a New American Security. "Producing and selling advanced military equipment is one way to signal to other states that you are not someone to mess with, nor are your allies someone they should mess with."

Selling these weapons is also lucrative. In a recent $2 billion deal, Almaz-Antey delivered 15 batteries of S-300PMU-2 mobile antiaircraft missile systems to China. Each battery has two or three radar units and four missile launchers. The radar can simultaneously track 100 targets; each launcher can shoot four missiles that speed toward targets at Mach 6. That's about 60 missile-launching vehicles for the price of four F-22 Raptors. The S-300's keen radar and fast-moving missiles guard the Taiwan Strait and form an umbrella that would protect a Chinese invasion of Taiwan.

Scary as the missiles might be, it's the radar systems that pose the gravest threat to stealth airplanes. Post–Cold War engineers in Russia breathed new, deadly life into VHF radars that have been around since the 1970s by digitizing their signals. Increasing computing power has improved the system's ability to glean coherent information from a jumble of data. Faint VHF radar returns that once would have been construed as random background noise can now be detected and identified.

"These VHF radars can detect aircraft constructed using stealth technology," Viktor Ozherelev, a division head at Almaz-Antey, claimed at a 2007 arms show. "The Americans know their stealth program has failed." Most experts say this is an exaggeration, but it's not unfounded.

The interplay between radar and airplanes is a physical one. Stealth airplanes are shaped to deflect radar waves away from the receivers—but not every radar scans at the same wavelength. Increasing the frequency of a wave decreases its wavelength (the distance between its peaks). The shorter the wavelength, the more detailed the return and the better the resolution.

Aerospace engineers designed stealth airplanes primarily to beat the detection equipment that poses the greatest threat—X-band radar. Surface-to-air batteries use this band because it operates at wavelengths that give the optimal compromise between the range and resolution needed to identify and track a target. But when stealth airplanes are exposed to radar waves longer than this wavelength range, they generate stronger radar returns.

For this reason, well-equipped defenders have more than one kind of radar protecting the same airspace, set up at different angles. For example, a defender protecting a fixed target (like a uranium-enrichment facility) could share data from a network of several radars to get enough information to accurately launch a missile. A VHF radar could detect incoming aircraft while lower-frequency stealth in these networked environments. "The rapid expansion of computing power ushers in new sensors and methods that will make stealth and its advantages increasingly difficult to maintain," Adm. Jonathan Greenert, chief of naval operations, wrote in the July 2012 issue of Proceedings magazine, published by the U.S. Naval Institute. "Maintaining stealth in the face of new and diverse counter-detection methods would require significantly higher fiscal investments in our next generation of platforms."

An anti-aircraft missile seeker displayed at an arms show outside Moscow. The 6-inch-diameter disc is studded with transmit/receive modules. The entire apparatus fits in the nose of a missile.

America's newest stealth aircraft, the F-35 Lightning II, is the most advanced warplane ever built. It's set to enter service in 2016, and at least eight nations are buying it, making this stealth warplane the most likely one to face Russian radar and missiles.

The F-35 diminishes its visibility to radar with internal weapons bays, carefully aligned edges, and embedded antennas. Yet the airplane is accused of being more vulnerable to detection than earlier stealth aircraft, such as the F-22 Raptor, due to its more conventional airplane shape. Air Force Association president, retired Lt. Gen. Mike Dunn, slighted the F-35 when he stated that "only the F-22 can survive in airspace defended by increasingly capable surface-to-air missiles."

The F-35 is a multirole aircraft; it must fight other airplanes, bomb targets, and conduct recon; and each mission requires specific payloads. For that reason, its design has tradeoffs that make it less stealthy and less maneuverable than the Raptor, which was designed first and foremost to win air superiority over other fighters.

The F-35 does not have the radar-shunting curves of the Raptor that help mask it from radar at all angles. Engineers designed the F-22 and the B-2 to be unseen at many wavelengths and directions. The Lightning II does not offer many radar returns when the waves strike it from the front, but when they come from the side, the returns are stronger.

Persistent F-35 critic Carlo Kopp, an analyst with the group Air Power Australia, has written that the Lightning II is "demonstrably not a true stealth aircraft." He also claims radar waves will bounce between the juncture of wing and fuselage in a way that can be detected if the airplane is scanned from any direction but the front. He is not the only one who has pointed out possible sources of trouble. For example, rival airplane-makers in Europe claim that powerful aircraft radar can spot an F-35 coming, even head-on, if multiple opposing aircraft are cooperatively scanning.

Radar waves do not just reflect off objects, they also flow across surfaces, scattering only when they hit a rivet, gun barrel, or other feature that breaks the smoothness of the skin. Aviation Week reporter Bill Sweetman notes that the F-35A's gun is located internally, but it is housed in a "hideous wart" on the airplane's surface—one of several features he says could betray the aircraft's position.

Lockheed Martin won't confirm or deny these alleged flaws, saying the information is classified. Still, the criticisms are plausible, even if they come from known F-35 skeptics using only public information. But Lockheed vice president and former F/A-18 pilot Steve O'Bryan pointedly notes that there is more to being low observable than just shape. "I reject the notion that the F-35 is an inferior stealth airplane," he says.

The F-35's approach to radar-absorbent material (RAM) is more reliable than that of any earlier warplane. The F-22's surfaces are made of aluminum, which are covered in RAM that must constantly be reapplied. This is, of course, a nightmare for maintenance crews. But the F-35 is made of carbon-fiber composite; Lockheed engineers bake RAM into the airplane's edges in an effort to soak up inbound radar.

But the Lightning II's key to survival is its own radar, the Active Electronically Scanned Array (AESA) installed in its nose. Conventional radar systems turn their gaze mechanically—imagine a dish spinning or a flat surface tilting to aim radar beams. Electronically steered radar does not move, but its beams can broadcast in different directions, thousands of times a second and across many frequencies. This agility allows AESA to map terrain and track hundreds of targets.

AESA is built to do more than scan—it can reach out to enemy radars and scramble their signals. A combination of radar and electromagnetic warning sensors alert an F-35 pilot to the threat of enemy radar; he can then dodge the threat or use the AESA to jam the signal, no matter what frequency the radar is transmitting.

And, if a missile is launched, the F-35 can track it with 360-degree infrared-sensor coverage and then, in some cases, overwhelm the missile's guidance system with the AESA. "Stealth works in conjunction with all those other techniques to make the F-35 what is probably the most survivable airplane of all time," O'Bryan says.

But there's a double edge to this sword. AESA radar is great at protecting stealth aircraft, but it can also detect them. Foreign military engineers are placing electronically steered radar arrays in their own warplanes and advertising them as stealth hunters.

Putin's radarmen are building several AESA radars for existing and future warplanes. Last year, Yury Bely, director of the Tikhomirov research institute, said in Takeoff, a Russian aerospace magazine, that the L-band AESA radar his staff is developing is "as good as any foreign radar of its type." This year, flights of an X-band AESA radar began in prototypes of the Russian—Indian PAK-FA stealth airplane.

Air dominance is now being fought in a greater swath of the electromagnetic spectrum. The critical part of any 21st-century air combat will be the first invisible duel of flickering AESA beams dancing across each other hundreds of miles ahead of any airplane. It's the same old dogfight rules: The first airplane to spot the other shoots, and quite likely whoever is in the other airplane dies.

Improving defenses are already influencing Pentagon attack strategy. No one is saying stealth designs should be abandoned, but military planners must reckon with the advances of opponents. This is another part of the game—one wonks call the cost—exchange ratio.

"There's a real risk that a lot of the high-tech investments are going toward things our adversaries want us to invest in, because they can neutralize them—and spend far less money to do so," Sharp says.

In his essay in Proceedings, Greenert suggests the way to keep U.S. aircraft safe is to make smarter choices in equipping them. Instead of investing massive amounts of money in airplanes that can defeat every new threat, he advocates purchasing weapons that existing aircraft can fire from longer ranges, safely away from radar. He also suggests adopting UAVs and missiles that can jam enemy radar before manned aircraft even arrive. "We need more numerous electronic warfare and cyber payloads to thwart detection and targeting," he wrote. "U.S. forces can... employ long-range sensor, weapon, and unmanned vehicle payloads instead of using only stealth platforms to reach targets."

The drones, cruise missiles, and decoys may lead an attack, spewing radar beams and flooding computer networks with viruses. But they will have to be built to defeat powerful radar networks and fast missiles with can't-miss seekers.

Years of fighting low-tech insurgencies have increased military reliance on UAVs that are easily spotted on radar. "We've got to start planning to build systems and to field capabilities to fight in a contested environment again," the Air Force's new military deputy for acquisition, Lt. Gen. Charles Davis, said in a recent interview with the Air Force Times. So, in one form or another, the duel will continue.

As of now, no S-300 surface-to-air weapon has ever been fired at a target in anger. The Lightning II is still in testing, and the debut of its style of networked warfare is years away. Their matchup awaits. Only then will the war of words between detractors, engineers, salesmen, and journalists be settled—in combat, with lives at stake and history in the balance.