SEN. JOHN GLENN (D-Ohio) achieved two unintended purposes by his much -discussed speech last week to the Veterans of Foreign Wars in New Orleans. He gave President Reagan grounds to sue him for plagiarism and he gave Democratic voters reason to doubt whether he offers much of an alternative.

The plagiarism was a piece of vintage Reagan. "Never again," Glenn told the VFW convention, "should we send American troops to fight wars we do not intend to win." This is a steal, slightly deodorized, from a famous Reagan remark to the same VFW's annual convention three years ago.

After calling the Vietnam war "a noble cause," Reagan then said we should "never again ask young men to fight and possibly die in a war our government is afraid to let them win."

This is one of the principal themes in the "basic Reagan speech" The New York Times compiled in March 1980 from scores of Reagan campaign addresses. It is a choice example of his genius for innuendo, implying cowardice and treachery, if not downright treason by persons safely unnamed.

Perhaps it was this purloined sentence which led The Los Angeles Times next day to report that Glenn "sounded like a younger Reagan without the rhetorical skills." Actually when the full text as delivered finally became available from Glenn's here, it sounded more like Calvin Coolidge. The speech might just as easily have been made by any Republican candidate for president manfully intent on carrying water on both shoulders.

As evidence, here is the sum total, verbatim, of what Glenn had to offer on the hottest topic of the day -- Central America. He approached the subject gingerly, observing (without too dangerous an originality) that unless we respond to the desire for change in the Third World, we must abandon it to extremists of right or left.

"Recently," the Senator then said, "the urgency of this choice has been hastened by the events unfolding in Central America. The question of American intervention there is now being very hotly debated. I cannot tell you how that debate will be resolved.

Glenn didn't say where he stood in the debate or how he would resolve it. He limited himself to telling the audience "what principles should guide our action." And these were? In each instance to find the proper mix of economic assistance and, if necessary, military intervention on "a case-by-case basis," but "force should only be used as a last resort." This is how the presidential ticket of Wintergreen and Throttlebottom boldly faced the issues in that old musical comedy, "Of Thee I Sing."

Of course Glenn was adjusting his tune to a right wing veterans' organization that is raising money for "freedom fighters" in Nicaragua. But Glenn's credentials as a veteran of World War II and Korea also made him an ideal person to try and talk sense to his old comrades in arms, instead of handing out mangy platitudes. He even told them -- who's writing his speeches? -- that it was "better to die on our feet than to live on our knees." Must he plagiarize McGuffey's Reader, too?

Glenn's identity crisis may be less important than his replay of Reagan's old line about the Vietnam war as if it were somehow an axiom of national policy, a venerable "lesson of Vietnam" -- especially when there have been quite a number of statements from Pentagon sources recently implying that the military do not want to get involved in Cental America unless this time assured of "a free hand." It is a chilling phrase.

This itself is another echo of the Reagan line. It has become a clich,e that had better be taken apart before it gets us into a lot of trouble.

Who ever heard of a government going into a war that it did not intend to win? The real question that often does arise as the war unfolds is whether pursuing it to the end would be worth the cost.

Of course such calculations disapppear when a nation is attacked and fights for its life. Typically this question tends to come up after a big power attacks a smaller one. This always looks easy when you compare the size and population of the attacker with that of the attacked.

The disproportion of forces can prove deceptive, as we learned in Vietnam and the Soviets are learning in Afghanastan. Of course a superpower can always crush a small power if it chooses to mobilize its full strength, ignore its security interests elsewhere, and risk widening international complications, and especially if it is prepared to destroy the smaller neighbor it claims to be liberating. The sobering question which crops up sonner or later is whether victory would be worth the full price.

The U.S. had to withdraw from Vietnam not because successive governments, Democrat and Republican, did not desperately want to win, but because they began to realize that victory, even if attainable, wouldn't be worth the cost. In Vietnam, as earlier in Korea, it was overwhelming American sentiment that decided against "winning."

In Central America now, supposedly routine naval maneuvers have already graduated to what a Pentagon spokesman last week called "polite harassment" of Soviet bloc vessels. What if someone gets impolite? The next step could be a blockade, and then? There must be more sensible and less expensive ways to ensure friendly neighbors.

Before an irreperable incident occurs, is it too much to ask Sen. Glenn -- and the other Democrats -- to speak up?