It's the sort of subject that the television networks customarily dump into a 10 p.m. time slot on a Friday during the August dog days to prove that they're paying their dues to public-affairs programming. But, for those viewers who want something a tad more substantial than summer reruns, the one-hour ABC News Closeup "Swords, Plowshares and Politics," on Channel 7 at 10 tonight, does offer a rough-cut try at explaining why the once-bright promise of the United Nations has been unable to prevent tragedies like the carnage now taking place in Lebanon.

Taking as their departure point the U.N. charter's pledge ". . . to end the scourge of war for all time," ABC's documentary expert, Marshall Frady, and correspondent William Sherman seek to look at the United Nations through the prism of three of its missions: international peace-keeping as exemplified by efforts in Lebanon, the safeguarding of human rights around the world and the care of refugees as illustrated by the situation in Honduras.

The score, as they tote it up, reads roughly like this: in Lebanon, a performance so ineffectual that it ultimately degenerated into irrelevance; in human rights, a cleaving to a double standard so blatant that it stands as a metaphor for collective human hypocrisy, and in refugee care, an "A" for effort and relatively high marks for effectiveness within the limits of what available resources and political realities will permit.

But, while Frady and his backup team do get those points across, they are less successful in conveying to the viewer a sense of why the United Nations' record is so erratically mixed. In part, that's because their approach -- using case histories of three of the United Nations' myriad functions -- fails to convey a sense of the vast and amorphous nature of the world body and the built-in structural stresses that perpetually force it to approach problems according to the lowest common denominator of member consensus.

This is crucial to better understanding on the part of an American public whose view of the United Nations ranges from glassy-eyed boredom to occasional bouts of distemper at the idea of its tax money helping to support what seems like a pulpit for foreigners to tell the world what's wrong with the United States.

Diplomats and journalists who deal regularly with the United Nations see it as something else--an organization that has been transformed from a relatively small club used by the United States and the Soviet Union in the Cold War era to score propaganda points off each other into a forum dominated by the Third World and regarded by it as its prime instrument for collectively confronting the industrial nations.

These are the reasons the United Nations International Force in Lebanon (UNIFIL) was blocked by pressure from the Arab countries and their allies from exercising the kind of control over the Palestine Liberation Organization that might have prevented bloodshed in Beirut. They are the reasons the United Nations is a place where South Africa's apartheid policies are denounced with monotonous regularity by governments that systematically band together to block any inquiry into their own abuses of human rights.

Frady does try hard to raise these points as he works through his case studies; and, at the end, he makes a stab at discussing them specifically. But, his preparation consists largely of uninspired stock footage and a kaleidoscope of fragmentary interviews that whiz by so fast that the viewer is left in confusion about who's talking and what subject is being addressed. In the end, insufficient time remains for anything other than a discussion too perfunctory to tie together for the novice viewer and put into perspective the bits and pieces of information that have been tossed out.

Perhaps the hour's most telling moment comes when a French officer attached to UNIFIL notes, almost as a throwaway line, "I think it's a big mess . . . that UNIFIL and the United Nations are a big mess. . . ."

Based on what ABC has shown him, the viewer seems certain to agree; but he's also likely to be left with the feeling that he's still not sure of the reasons. It might not be a bad idea if ABC and Frady were to step back and, building on what they've already done, take another crack at telling him why.