After Lebanon, US must redefine 'vital national interest'

Washington is enjoying a much-needed pause for reflection about the matter of using military power in support of foreign policy. The affair of Lebanon underlined the importance of a better understanding of how, when, in what way, and under what conditions it can be used to advantage.

Lebanon was a case in which President Reagan wanted a strong, united, and pro-Israel ally. He sent 1,800 Marines to the coastal plain of Lebanon to that end. He called Lebanon a ''vital national interest.'' He said that failure to set up a strong, united Lebanon could have ''disastrous results worldwide.''

Well, over the past week the Lebanon operation wound down. The Marines were offshore with the fleet, except for an embassy guard and a training mission for the Lebanese Army. President Gemayel went to Damascus and, in effect, bent the knee to Syria's President Assad. The ''Syrianization'' of Lebanon was under way.

Were there ''disastrous worldwide consequences''? If so, they were not evident. Had a ''vital national interest'' been lost? If so, no one seemed to notice.

Rhetorical inflation of the importance of imposing an American preference on Lebanon caused an unnecessary humiliation. A truly ''vital'' interest is something a country is ready and willing to fight for to the death. What is ''vital'' to the United States?

Friendly neighbors to the north and south are vital. Control of air and sea lanes to allies in Europe and Asia are probably vital. Some would say that Central America is vital. The survival of a free and friendly Western Europe would be high on the list.

The US would fight for any of those places or conditions, and would have the support of Congress and public opinion in doing so. But equally obviously, what happens in Lebanon is not on the same scale. The US will continue to be an influence in the Middle East in spite of Amin Gemayel having to accept his authority from Damascus rather than from Washington.

The main meaning of the loss of Lebanon to Syria is that someday the US will find itself negotiating with Syria. A continuing American goal in the Middle East is a comprehensive and lasting peace between Israel and its Arab neighbors. Syria has become the most important and influential of those neighbors. There can be no comprehensive and lasting peace between Israel and its neighbors now without the participation of Syria.

This is an inconvenience for Washington. Had the policy of setting up Mr. Gemayel as the leader of all of Lebanon succeeded, Syria's interests would have been less important. A lower price would have had to be paid for Syrian acceptance of peace with Israel.

One anxiety at the White House during all this was that failure in Lebanon would mean less respect for the United States in other parts of the world. The US has enormous military power. Is it unable to use it in places like Lebanon just because Congress and public opinion wanted the Marines out?

The key fact in the Lebanon story is that Congress and public opinion did not perceive Lebanon as being a truly ''vital'' national interest. The assertion that it was vital was unconvincing. Perhaps the most important lesson is that Mr. Reagan can use the armed forces of the United States effectively and decisively without first convincing public opinion and Congress that the issue is truly vital.

Another lesson is that when operating in parts of the world as complex and remote from normal American life as the Middle East, it is prudent to employ people with knowledge of the area. A friendly Western diplomat commented that the Lebanon operation was run without a single person in the higher levels of planning and decision who had ever served in the Middle East.

That absence of area knowledge probably explains why it was assumed that a combination of 1,800 Marines plus the Israeli armed forces would be sufficient to persuade Syria to do what Washington wished it to do.