It's not an easy time to be an American abroad. Or allies maintain discreet distance from us, giving lukewarm support in public about our ordeal over Iran but tut-tutting somewhat gleefully in private about how far the mighty Americans have fallen. Our good neighbor to the south, Mexico, suddenly closes the door at a critical moment, adding to our sense of isolation. Our hopes, if any, for real help from the United Nations remain modest indeed; it takes that august body a full month even to get around to considering so elemental a threat to all diplomats and all national sovereignty. Everywhere, it seems, you hear pitying but smug (if not secretly pleased) remarks about America's weakness, America's lack of resolve, America's impotence.

"Look what happens," said a financier at dinner. "Your embassy is burned and two Marines are killed in Pakistan, and you do nothing." And even some of our own most respected commentators bemoan what they believe to be American decay. George F. Will sees "Vichyite behavior" in the actions of some of our released embassy hostages and pronounces it "not the result of two weeks of captivity in Iran, but of years of absorbing the spirit of a liberal culture." He adds: "There is too much of the France of 1940 in the United States of 1979."

Here, at least, is one American who bridles at all these assertions. The history of these last 40 years testifies to quite a different reading of America's willingness to sacrifice to both its resoluteness and exercise of responsibility in the world.

Strolling through Mayfair the other day I passed an old church with a tablet embedded in the stone walls. It was not, as I first assumed, a memorial to some ancient event. "Inside this church," it read, "the armed forces of the United States of America prayed for divine guidance during the war years 1939-45 and gave thanks for the victory of the allied powers."

There weren't many Americans in Britain when World War II began 40 years ago and at home the United States was wrestling with whether to remain isolationist or become more active internationally. The outcome was uncertain. Even a year later, when events worsened and war for the United States became a virtual inevitability, our armed forces were conducting maneuvers with wooden rifles and we were spending less than $1.5 billion for national defense. Authority to draft American citizens was achieved by only a one-vote margin in Congress. Some of the nation's most noted names were warnng against American involvement in any European wars.

In the decade since,the United States has assumed perhaps more international burdens than any other society in history. The expenditure of lives and national treasure, given freely by the citizenry over so long a period, has been unprecedented.

We left nearly half a million dead out of our armed forces of 16 1/2 miion during the war. The cost in dollars alone was more than 250 billion. After the war, we rebuilt out former enemies' industrial plants from the ground up. We provided the economic salvation for Europe through the expenditure of more billions in the Marshall Plan. We served as the military shield for Europe, employed a nuclear umbrella of defense and stationed hundreds of thousands of American troops on the line between East and West (and 300,000 of them are still there).

And all during that period Americans spent the greatest amount of their national resources for defense and the air of countries around the world -- and they contnued to die in service abroad. Seven million Americans served for the three years of the Korean War, and 55,000 of them died.

After that, we maintained a standing armed force of some 3 million, dispatched the fleet to the Mideast, the Marines into Lebanon, and stood watch around the world during the years of peace between Korea and Vietnam. Then, for nearly 12 years, we had some 10 million people under arms as the initial trickle of American blood in Southeast Asia turned into a hemorrhage. This time, 57,000 more Americans died in service and another hundred thousand became casualties. We are still paying the economic price of that drawn-out and tragically misguided conflict in the form of an inflationary spiral that began in the early massive mobilization over Vietnam in the mid-1960s.

I don't recite all this to strike a jingoistic note. But I do insist that the record of all the years from the beginning of World War II to the present stands as a source of national pride. And it marks a historic change: from the days of the first (neutrality Act in 1794, stating U.S. desire to keep out of war between France and England, the American people and their policymakers traditionally shunned a military role abroad. These last 40 years have ended that forever.

Someone asked me, somewhat belligerently, during this brief visit to London, if it weren't true that Americans had become too soft to fight. I replied, with some heat of my own, that such a view was nonsense.

The real question is different. Dr. Johnson was right, as always, and probably never more so than in his remark about patriotism being the last refuge of scoundrels. But there are many forms of patriotism. The easiest, and cheapest, as old Sam knew well, involves waving the flag and blaring the trumpets. The more difficult requires exercising restraint in the face of flagrant provocation and yet remained measured and strong. Which was just the example President Carter set in his news conference this week.

He was not mistaking lack of military action for weakness and neither, I suspect, are the American people. In the perspective of America's acts during these difficult last two generations, the present national response to the agonizing crisis in Iran should bring a renewed pride in the country.