The North Atlantic Treaty Organization, the military alliance that has kept peace in Europe since 1949, is observing its 40th birthday.

Unhappily, this particular milestone finds NATO caught in more crises rather than enjoying more prog-ress.But then the crises arise because in some ways NATO is the victim of its own success.

One crisis is because NATO hasn't figured out how to respond to the peace offensive launched by Soviet leader Mikhail Gorbachev. Another problem results from Europe's increasing economic unity and prosperity, which can deprive the United States of its accustomed leadership role at a time when no European power appears ready to take up that role.

The situation has gone so far that there is increasing talk about the possibility of the United States starting to withdraw some of the 350,000 troops it has stationed in Europe as part of its commitment to NATO. Such talk is being heard just as much in Washington as it is in Europe.

Up to a point, American dissatisfaction with NATO is understandable. Our European allies never have pulled their own weight in the alliance. The United States now spends $160 billion a year in defense of Europe - more than the rest of the NATO allies combined. A case can be made that the money saved by the Europeans has been used to clobber their American benefactors in trade wars.

European ingratitude is reflected in other ways, too. One that particularly rankles is the failure of many European leaders to support U.S. security interests outside Europe. For example, the NATO allies initially refused to help American forces in the Persian Gulf in 1987 and some have given financial aid to Nicaragua's Sandinista government even though the U.S. has vigorously opposed the Sandinistas.

But if the U.S. starts pulling out of Europe, it will lose even more influence there than it already has lost. What's more, some European leaders seem determined to seize on the withdrawal of even a few U.S. troops as an excuse to lower their own guard, too.

Down that road lies potential disaster. As it is now, NATO forces face a communist bloc with much more military manpower. A weaker Europe could easily invite renewed Soviet aggression. The West can't count on Gorbachev's more moderate policies, let alone Gorbachev himself, prevailing indefinitely. The changes imposed by the present leader of the Kremlin could easily be reversed by whoever succeeds him. In fact, even Gorbachev didn't start agreeing to moderate its military posture until after the U.S. insisted on bolstering its missiles in Europe and the lopsided Soviet economy, with its great overemphasis on military spending, turned sour.

By all means, NATO should reassess its role in light of changing circumstances. But the reassessment should be toward more flexibility and a more equitable sharing of the burden, rather than in the direction of weakening the organization that has helped keep Western Europe free and safe for four decades.