BRUSSELS — Defense Secretary Robert M. Gates bluntly criticized NATO nations on Friday for what he said were shortages in military spending and political will, warning of “a dim if not dismal future” unless more member nations scaled up their participation in the alliance’s activities.

NATO has struggled for a generation to define its place in a post-cold war world, and its member nations have frequently quarreled about the scope of the alliance’s commitments and their individual responsibilities.

With little indication of any change in policy among the more reluctant member nations — notably Germany, the Netherlands, Poland, Spain and Turkey — Mr. Gates’s harsh words seemed likely to increase pressure on an alliance already deeply strained by differences over sharing the burden in Libya and Afghanistan.

Perhaps most significantly, Mr. Gates issued a dire warning that the United States, the traditional leader and bankroller of the alliance, is exhausted by a decade of war and and its own mounting budget deficits, and simply may not see NATO as worth supporting any longer.