Secretary of Defense Robert Gates did not mince words in Europe on Friday where he urged the allies to improve their own defense capability as the United States might no longer be willing to act as the continent’s security guarantor in the near future. “The kind of emotional and historical attachment” to NATO, he said, “is aging out.”

Gates, who is due to retire as defense chief this summer, warned that “future American political leaders — those for whom the Cold War was not the formative experience that it was for me — may not consider the return on America’s investment in NATO worth the cost.”

For many decades, extravagant American defense spending has compensated for repeated military cutbacks across Western Europe. The explicit American security guarantee embedded in NATO is part of the reason for the European allies to spend far less on defense than is required by treaty; the lack of a credible threat within the European Union being another.

Complains of Europe “free riding” on American power are nothing new but according to Gates, there is a “dwindling appetite and patience in the US Congress” to continue to make up for Europe’s lack of an independent defense capacity. Politicians and public alike, he said, are increasingly resistant to spending “precious funds on behalf of nations [that] are apparently willing and eager for American taxpayers to assume the growing security burden left by reductions in European defense budgets.”

Whereas during the Cold War, the United States accounted for roughly 50 percent of military spending within NATO, since the collapse of the Berlin Wall, that share has risen to more than 75 percent. In the face of huge deficits, European countries are expected to trim their defense budgets even further. America, too, will have to rein in defense spending. Gates has proposed some $400 billion in cuts over the next decade.

Only five out of 28 NATO member states spend more than the required 2 percent of gross domestic product on defense — Albania, France, Greece, the United Kingdom and the United States.

Britain and France, Europe’s two largest military powers, pushed for the military intervention in Libya but despite their enthusiasm, a quick and powerful American offensive was necessary to clear the ground for the enforcement of a no-fly zone. “NATO’s serious capability gaps and other institutional shortcomings [were] laid bare by the Libya operation,” according to Gates.

The mightiest military alliance in history is only eleven 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.

The defense secretary reportedly criticized Germany and Poland, which haven’t participated in the mission at all, behind closed doors and urged the Netherlands, Spain and Turkey to step up their role and start conducting airstrikes. He did have praise for Denmark and Norway. The two Scandinavian countries “have provided 12 percent of allied strike aircraft yet have struck about one third of the targets,” he said. “Belgium and Canada are also making major contributions to the strike mission.”