To understand just how and why we now have US marines under fire from Druze militiamen in the mountains overlooking the coastal plain south of Beirut, and returning that fire, it is well to look back to why and for what declared purpose the marines went to Lebanon.
A year ago, beginning on Sept. 16, a dreadful event occurred in Lebanon. Lebanese Phalangist militiamen, sent by Israeli forces into two refugee camps filled with Palestinians, began a massacre which went on unchecked for two days. Word of it leaked out of the camps beginning on Sept. 17. The world was shocked. On Sept. 18 President Reagan was ''horrified'' and expressed ''outrage.''
On Sept. 20 the Lebanese cabinet requested the return to Lebanon of a peacekeeping force of US, French, and Italian troops. On that same day President Reagan went on national television and said:
''The scenes that the whole world witnessed this past weekend were among the most heartrending in the long nightmare of Lebanon's agony.
''The international community has an obligation to assist the government of Lebanon in reasserting its authority over all its territory. With this goal in mind, I have consulted with our French and Italian allies. We have agreed to form a new multinational force similar to the one which served so well last month, with the mission of enabling the Lebanese government to resume full sovereignty over its capital. For this multinational force to succeed it is essential that Israel withdraw from Beirut.''
Israel objected, but under ''intense pressure'' from the US finally agreed to the redeployment of the multinational force. The first contingent of 350 French paratroopers arrived on Sept. 24. US marines arrived on Sept. 29 after Israel forces had withdrawn from the area assigned to the marines.
This week, a year later, the US marines are still there and still have the official mission of assisting the government of Lebanon ''in reasserting its authority over all its territory.''
But the problem blocking the assertion of the authority of the government of Lebanon has changed. The Israelis have withdrawn from Beirut itself and from the area south of the city. They have established a new line about halfway down from Beirut to the frontier of Israel itself.
The area from which the Israelis have withdrawn is inhabited largely by the Druzes in the mountains, and along the coastal plain by Shiite Muslims, the economically poorest of the various groups who make up the heterogeneous population of Lebanon, and by the residue of Palestinian refugees.
The Israeli withdrawal disclosed the fact that the Druze and Shiite communities do not accept the existing government of Lebanon as being fairly representative of their interests. They are resisting the attempt by the armed forces of that government to assert its authority. They are aided and backed in their resistance by Syria.
The government of Lebanon is dominated by the family of Amin Gemayel which also controls the Phalangist militia. The Gemayel family is the leading influence in the Maronite Christian community. The Maronites are the economic and cultural elite of Lebanon. They are a minority of the population controlling a majority of the wealth and political power. They are associated with Israel. Their Phalangist militia was for long armed, paid, and trained by Israel.
The original idea behind sending in the marines was that if Israel and Syria would withdraw from Lebanon the various fragments of the Lebanese community would come together under a Lebanese government. The marines would help to police the process.
But the Gemayel faction failed to come to terms with the Druzes and with the other Muslim elements. In the eyes of the others the US marines are now attempting to impose a Maronite Christian dominance upon the whole country. The Druzes are not shooting at the marines because they are US marines but because they are associated with the Maronite Christians who have come to terms with Israel.
In other words, the marines are in fact caught up in a civil war in which the Muslims are fighting against domination by the Maronite Christians.
This is not the purpose behind the original deployment of the marines to Lebanon. Should they be there to help one faction prevail over the others?