THE debate about President Clinton's planned deployment of American troops to Bosnia adds prominence to a question that is likely to arise again and again in the post-cold-war era: How does the United States react to a crisis that is neither vital nor irrelevant to US interests but somewhere in between?
Congress will almost certainly acquiesce, albeit grudgingly, to Mr. Clinton's determination to dispatch 20,000 US soldiers to help guarantee a Bosnian peace treaty. But unless the mission is an unqualified success - which is an unlikely prospect - it may not establish clear guidelines for the future.
That could be a fact of considerable consequence as the US pursues its quest to extend the protection of NATO to nations in Central Europe that, historically, have not been vital to US security.
''If Congress is having this much trouble sending troops to Bosnia - in a role that involves a minimum amount of risk - how serious is its willingness to back up the commitment to defend the territorial boundaries of the countries earmarked for NATO membership?'' asks Charles Kupchan, senior fellow for Europe at the Council on Foreign Relations, in New York.
''The American public is demonstrating its unwillingness to pay a price of involvement in Central Europe, and this illustrates an explicit or implicit condition that will have to be placed on the expansion of NATO guarantees to new members,'' concurs Barry Blechman, chairman of the Henry L. Stimson Center, in Washington.
The Clinton administration and apparent majorities in both houses of Congress support the idea of granting full NATO membership to several Central European nations, including Poland, Hungary, and the Czech Republic.
New members will be admitted individually, subject to confirmation in the parliaments of existing member nations. Analysts say the Bosnia experience could add a note of realism to eventual deliberations in the US Senate.
Former Defense Secretary Harold Brown notes that many lawmakers have seen NATO expansion as a ''free good,'' providing the benefits of incorporating former Soviet bloc nations into the Western orbit without cost.
''Having to expose American troops to the possibility of being attacked in Bosnia concentrates the mind on the question of whether expanding our commitments increases the possibility of the same thing happening somewhere else,'' says Mr. Brown, now at the Center for Strategic and International Studies in Washington.
NATO defense guarantees are mainly designed to cover cases of outright aggression. But Bosnia-type situations, though far less likely, could arise. One hypothetical example: a conflict erupts between the Slovakian government and its Hungarian minority, backed by arms, money, or supplies from Hungary.
In either case, the question that now hovers over the Bosnia debate would be raised again: Are American interests sufficiently at stake to warrant US intervention as part of a NATO-led force? Analysts note the irony that some of the lawmakers who are the staunchest supporters of NATO expansion are the staunchest opponents of Clinton's plan to deploy US troops in Bosnia.
For some there is no paradox involved. The whole point of extending NATO's protective umbrella, they say, is to ensure that there are no more Bosnias in Europe. If the threat of Western intervention had been more explicit four years ago, they note, the Bosnia crisis might have been avoided.
But some analysts note a disconnect between the ambitious aims of NATO and US reluctance to pay the price of leadership.
''Support for NATO expansion is entirely schizophrenic,'' notes Mr. Kupchan. ''People mostly support it for ideological reasons: a residual anti-Russian sentiment mixed with a justifiable desire to attach the central European countries that suffered under communism to the West. But they are the same people who are stingy about putting American lives and resources on the line.''
Historically, the US has intervened in Europe only to keep a single powerful nation - like Hitler's Germany - from upsetting the balance of power, extending its hegemony across the Continent.
''There's been a single thread of American commitment to Europe since 1917 and it has brought us into conflict with three different regimes: Wilhelmine Germany, Nazi Germany, and Soviet Russia,'' notes Charles William Maynes, editor of Foreign Policy magazine.
In the absence of a European power with hegemonic aspirations, Mr. Maynes says, Clinton must change the rationale for US intervention from deterring aggression to guaranteeing Europe's domestic order. ''That's a new task, one that we've never assumed before.''
BUT it is one the US could be called upon to assume again and in places besides Europe, since social disorder, not aggression, has become the primary threat in the aftermath of the cold war. And out of social disorder has come what diplomatic historian John Lewis Gaddis describes as a decentralization of genocide.
''Horrors can arise because people like Hitler order them to take place, but horrors can also percolate up from below,'' says Mr. Gaddis, who directs the Contemporary History Institute at Ohio University, in Athens, Ohio. ''If you look at what's happened in Yugoslavia, it's a democratic form of genocide that's set in motion because people hate each other so much.''
Diplomatic analysts conclude that the Bosnia experience points to the narrowly defined circumstances under which an American president can commit troops in circumstances where US interests are not directly threatened, as they were when Iraqi forces occupied Kuwait's oil fields in 1990.
''If the intervention succeeds, what is the lesson? That if two groups fight one another long enough and divide territory and exchange populations, then a line can be established between them that an outside power can confirm,'' Maynes says. ''If the mission is to protect an ethnic minority in hostile territory, it is mission impossible.''