The Kosovo war revealed a profound gap between the military capabilities of the United States and its European allies that could soon lead to serious friction over how to share defense burdens, according to senior NATO officials and diplomats.

The triumph of the air campaign against Yugoslavia was tempered at NATO headquarters by the stark realization that Europe has fallen so far behind the United States in the use of precision-guided weapons, satellite reconnaissance and other modern technologies that the allies are no longer equipped to fight the same way.

In more than a dozen interviews, NATO political and military authorities said that the lopsided division of labor between the United States and Europe in the air war demonstrated that the alliance is in danger of evolving into a two-tier organization, with gross inequalities of military might that may distort NATO's ability to cope with crises.

In the Kosovo conflict, the United States -- which spends nearly four times as much as its European allies on defense research and development -- supplied more than 80 percent of the aircraft and nearly all the intelligence resources used to select bombing targets. Meanwhile, the Europeans were relegated to flying mop-up missions, providing host-nation support and deploying the bulk of the 50,000 ground troops in the NATO-led peacekeeping force.

Partly to compensate for the overwhelming military role played by the United States, the 15 European Union countries have vowed to pay for most of the economic reconstruction of Kosovo and the rest of the Balkans. German Foreign Minister Joschka Fischer said the rebuilding plan could cost $30 billion over the next six years.

"The Kosovo war was mainly an experience of Europe's own insufficiency and weakness; we as Europeans never could have coped with the Balkan wars that were caused by [Yugoslav President Slobodan] Milosevic without the help of the United States," Fischer said. "The sad truth is that Kosovo showed Europe is still not able to solve its own problems. We have to accept the consequences and hope that Europe can grow from this crisis."

NATO Secretary General Javier Solana, who will become the European Union's foreign policy czar later this year, said his biggest challenge will focus on a European security policy and on persuading EU governments to reshape their defense forces in ways that are compatible, not competitive, with those of the United States.

"We do not need to build a second NATO," Solana said. "It's a matter of political will and harmonizing Europe's military industries, but most of all it's a matter of money. It's hard to say just how much will be enough. Defense budgets will have to rise, but we could accomplish a lot just through better coordination of the way we spend our money."

But at a time when there is no strategic threat to NATO and European voters are being asked to surrender cherished social entitlements, it will be politically difficult to justify a new surge in defense spending.

"Everyone agrees on the lessons to be learned from the Kosovo experience, but few people here are confident that we will apply them," said a senior NATO intelligence official. "It would take the Europeans two decades to catch up with the Americans even if they had the money and the will to spend it."

In his valedictory address last month, German Gen. Klaus Naumann, who was retiring as chairman of NATO's military committee, said the Kosovo conflict confirmed his worst fears that the day is fast approaching when the United States and its European allies "will not even be able to fight on the same battlefield."

The air war demonstrated the result of a U.S. investment in military research and development that has dwarfed Europe's in recent years. The United States now devotes $35 billion a year to creating the kinds of advanced weapons and intelligence gathering systems used over Yugoslavia, while European alliance members spend $10 billion a year -- a sum fragmented into national projects that shrink its impact.

Since the end of the Cold War, European governments have slashed their defense budgets -- in some cases, almost in half. Austria, an EU member that has rejected appeals to join NATO because of its neutral tradition, now spends more money on its state opera company than on national defense.

Among the 11 EU states that belong to NATO, many have been reluctant to invest in transforming their armed forces from large standing armies designed to thwart a Soviet-led invasion into the mobile, flexible units needed to cope with new security threats beyond their frontiers, such as the Kosovo crisis.

Despite some progress in reorganizing their militaries, the European allies have balked at making key decisions that would improve their ability to cope with Kosovo-like situations. They spurned an opportunity to buy at a bargain price the U.S. J-STARS air-to-ground surveillance system that kept NATO planes out of harm's way over Yugoslavia. Because of disputes over how to share jobs and costs, France and Germany aborted plans to build a satellite reconnaissance system and a heavy-lift transport aircraft that would have made them much less dependent on U.S assets.

Air Force Lt. Gen. Michael C. Short, who orchestrated the NATO bombing campaign, said the shortcomings of European aircraft were so glaring -- such as the lack of night-vision capability and the absence of laser-guided weapons systems -- that he curtailed their missions to a minimum to avoid unnecessary risks. Short said that unless remedies are found soon, the alliance will be riddled with "second- and third-team members" incapable of flying the same missions as U.S. forces.

The discrepancy in American and European military capabilities also threatens to affect NATO's ability to shape consensus on combat strategies. While cognizant of the need in an alliance of democracies to obey the will of their political leaders, Short and U.S. Gen. Wesley K. Clark, NATO's supreme commander, acknowledged that military strategists would consider the Kosovo air war a textbook example of how political constraints can undermine a conventional bombing campaign.

As an airman, Short said he would have "gone downtown on the first night" and taken the war to Yugoslavia's civilian population by knocking out bridges, power plants and telephone networks. But France and other European governments vetoed many civilian targets and imposed a limited, phased approach that the military commanders say delayed victory.

In the end, Short and Clark say, it was NATO's ability to hit "strategic, fixed targets" -- causing an estimated $30 billion damage and widespread hardship among civilians -- that ultimately compelled Milosevic to accept the alliance's demands. Tactical raids against Serb-led Yugoslav military forces, which NATO commanders now say were less effective than they believed because of the use of ground decoys, apparently had a negligible impact on the Belgrade leadership until the war's closing days.

As NATO conducts its post-mortem of the Kosovo conflict, NATO military officers insist that if the alliance hopes to prevail in similar operations short of full-scale war, they will need to retain control of the key elements of surprise and maximum firepower, which they were denied in the campaign against Yugoslavia.

Naumann and Clark also have recommended that NATO take a hard look at its crisis management methods. The obsession with sustaining consensus within the alliance meant that NATO was unprepared when Milosevic accelerated the violent expulsion of Kosovo's ethnic Albanian majority as the NATO bombing began.

The alliance was caught wrong-footed as the massive exodus of refugees threatened to destabilize neighboring Albania and Macedonia. It also made a mockery of NATO's claim that it launched the air war to prevent a humanitarian catastrophe, when it looked as if the bombing only made matters worse for the Kosovo Albanians.

"We were much too narcissistic," said a senior NATO planner. "This air war was prepared almost as if Milosevic did not exist. We thought he would buckle right away, and when he didn't we did not know what to do except keep on bombing. What the alliance needs in dealing with future conflicts are more chess players and fewer pollsters."