BRUSSELS >> In a stern rebuke, U.S. Defense Secretary Robert Gates warned Friday that the future of the historic NATO military alliance is at risk because of European penny-pinching and distaste for front-line combat. The United States won’t carry the alliance as a charity case, the outgoing Pentagon chief said.

Some NATO countries bristled, but Britain quickly and heartily agreed.

Gates’ assessment that NATO could face "a dim if not dismal" future echoes long-standing concern of U.S. policymakers about European defense spending. But rarely, if ever, has it been stated so directly by such a powerful American figure, widely respected in the United States and internationally.

The remarks, at the close of Gates’ final overseas trip, reflect a new reality of constrained American finances and a smaller global reach.

Earlier in the week Gates played "bad cop" to U.S. President Barack Obama’s good, criticizing Germany’s abstention from the air campaign in Libya two days after Obama lavished an award and fancy White House dinner on visiting Chancellor Angela Merkel.

But Gates spoke for the Obama administration, and his warning Friday was aimed squarely at Europe’s priorities.

"The blunt reality is that there will be dwindling appetite and patience in the U.S. Congress, and in the American body politic writ large, to expend increasingly precious funds on behalf of nations that are apparently unwilling to devote the necessary resources or make the necessary changes to be serious and capable partners in their own defense," he said.

That assessment may cause Europeans to question the future of their defense relationship with the United States, on whom they have counted for a large measure of their security for six decades.

It comes on the heels of the withdrawal of one American combat brigade from Europe as part of a significant reduction of U.S. troops in Europe.

The U.S. has been the brawn behind NATO since its birth in 1949. But the disparity between strength and allies’ investment has only grown wider.

In a question-and-answer session after his speech, Gates, 67, said his generation’s "emotional and historical attachment" to NATO is "aging out." He noted that he is about 20 years older than Obama, his boss.

For many Americans, NATO is a vague idea tied to a bygone era, a time when the world feared a Soviet land invasion of Europe that could have escalated to nuclear war. But with the demise of the Soviet Union in 1991, NATO’s reason for being came into question. It has remained intact — and even expanded from 16 members at the conclusion of the Cold War to 28 today — but European reluctance to expand defense budgets has created what amounts to a two-tier alliance: the U.S. military at one level and the rest of NATO on a lower, almost irrelevant plane.

Gates said this presents a problem that could spell the demise of the alliance.

"What I’ve sketched out is the real possibility for a dim if not dismal future for the trans-Atlantic alliance," Gates said. "Such a future is possible, but not inevitable. The good news is that the members of NATO — individually and collectively — have it well within their means to halt and reverse these trends, and instead produce a very different future."

Without naming names, Gates blasted "nations apparently willing and eager for American taxpayers to assume the growing security burden left by reductions in European defense budgets."

A German foreign ministry spokeswoman defended that nation’s contribution and noted Obama’s recent praise.

However, defense spending is uneven within Europe.

Liam Fox, defense secretary in Britain, a strong U.S. ally, told NATO Thursday that European governments were undermining military co-operation with the U.S. by failing to spend enough on defense. He also said other European nations should be more willing to send their forces to NATO operations such as Afghanistan.

He praised Gates as a champion of the trans-Atlantic relationship.

"Unless Europe carries more of the share of its own defense, we should not assume his successors will do the same," Fox said.

Over the past two years, military spending by NATO’s European members has shrunk by about $45 billion — the equivalent of the entire annual defense budget of Germany, one of the alliance’s top-spending members.

As a result, the U.S. defense budget of nearly $700 billion accounts for nearly 75 percent of the total defense spending by NATO members. The combined military spending of all 26 European members is just above $220 billion.

The White House stood by Gates’ comments Friday, though officials emphasized that the outgoing defense secretary was not guaranteeing a dim future for NATO, only saying that the possibility existed if allies cannot provide the resources needed. "I don’t think anyone would argue with that," said Tommy Vietor, spokesman for the National Security Council.

Gates has criticized the Europeans before. He bruised feelings at NATO by publicly calling for larger troop contributions in Afghanistan. He has also criticized the heavy restrictions many European governments set for their soldiers, including bans on night patrols that mean many of them rarely leave their bases.

In February 2010 at the National Defense University in Washington he said NATO was in danger of becoming a paper tiger.

"The demilitarization of Europe, where large swaths of the general public and political classes are averse to military force and the risks that go with it, has gone from a blessing in the 20th century to an impediment to achieving real security and lasting peace in the 21st," he said then.

To illustrate his concerns about Europe’s lack of appetite for defense, Gates pointed to Libya, where France and other NATO nations pushed hard for NATO intervention and where the U.S. insisted on a back seat role.

"While every alliance member voted for the Libya mission, less than half have participated at all, and fewer than a third have been willing to participate in the strike mission," he said. "Frankly, many of those allies sitting on the sidelines do so not because they do not want to participate but simply because they can’t."

Such inequality is unacceptable, Gates said, and so is the poor follow-through that occurred once the mission began.

"The mightiest military alliance in history is only 11 weeks into an operation against a poorly armed regime in a sparsely populated country, yet many allies are beginning to run short of munitions, requiring the U.S., once more, to make up the difference," he said.

During his first two years on the job, Gates alternately coaxed and complained, often loudly pressing allies to send more forces and funding to Afghanistan and to lessen their restriction on the troops they had there.

After a while he scaled back his constant hounding, acknowledging that it wasn’t paying off much. And he frequently joked that NATO colleagues weren’t shy about mentioning his "megaphone diplomacy."

NATO did send more forces over the past two years, and Dutch, British and other European forces have taken heavy losses. But as the Afghan war approaches its 10th anniversary, the U.S. has more than twice as many forces in Afghanistan as all other nations combined. Several NATO nations have withdrawn forces or have announced plans to do so. The U.S. shares the NATO goal of ending combat there by 2015.

Gates offered praise and sympathy along with his chiding, noting that more than 850 troops from non-U.S. NATO members have died in Afghanistan. For many allied nations these were their first military casualties since World War II.

Gates spoke at the Defense and Security Agenda think tank in Brussels, where earlier in the week he attended a two-day meeting of NATO defense ministers.