BRUSSELS — Alarmed by years of cuts to military spending, the NATO secretary general, Anders Fogh Rasmussen, issued a dire public warning to European nations, noting that together they had slashed $45 billion, or the equivalent of Germany’s entire military budget, endangering the alliance’s viability, its mission and its relationship with the United States.

That was two years ago. Since then, with the Afghan war winding down and pressure from the European Union to limit budget deficits, Europe has only cut deeper.

Now, as President Obama wrestles with his own huge budget deficit and military costs, the responsibility for keeping NATO afloat has fallen disproportionately onto the United States, an especially untenable situation as priorities shift to Asia.

The United States finances nearly three-quarters of NATO’s military spending, up from 63 percent in 2001. And yet among the alliance’s 28 nations, experts note, only the United States, Britain and Greece are meeting NATO’s own spending guidelines of 2 percent of gross domestic product. Even Britain and France — the two leading European nations willing to project military might — are slipping further. France says that by 2014 it may cut deeper still — to just 1.3 percent of G.D.P., down from 1.9 percent this year. By comparison, the United States spent 4.8 percent of its G.D.P. on the military in 2011.