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 US 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 US 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 US, 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 US 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 US 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-US 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.