President Reagan's decision to return U.S. Marines to Lebanon represents a gamble that the troops, along with French and Italian soldiers, can not only end the killing in that country but also save the president's long-range Middle East peace initiative from becoming a war casualty.

Officials here see the overall peace proposal announced by the president Sept. 1 is the most important foreign policy initiative of his administration. Aside from dramatizing the U.S. role as the region's only potential peacemaker, the plan is seen as crucial politically at home and diplomatically abroad because it poses a solution to the Israeli-Palestinian issue that seems plausible and has won respect for the president in many quarters.

In the last week, however, administration officials became increasingly convinced that the rapid, surprising and often gruesome events in Beirut -- Israeli occupation of the city, assassination of the new Lebanese president by unidentified assailants and massacre of Palestinian refugees, apparently by Christian militiamen -- were dealing a death blow to the peace plan.

And it was felt that the Israeli government of Prime Minister Menachem Begin, which rejects the Reagan peace plan, was knowingly contributing to its demise by its actions in Lebanon, particularly the invasion of West Beirut that paved the way for Christian militiamen to enter the region.

Thus came the gamble on sending in the Marines again, sharply worded warnings to Israel to get out and the president's second televised address on the crisis within three weeks.

The gamble associated with sending in the Marines this time, in contrast to their 16-day mission that ended earlier this month in Beirut, is that the current mission is more open-ended and not as well-planned.

In August, the multinational force was inserted to oversee a specific event -- withdrawal of Palestine Liberation Organization fighters from a city then only ringed by the Israeli Army.

Under the new plan, the troops will enter for what the president says is a "limited period of time." But officials acknolwedge that they do not know how long it will take the Lebanese government to achieve some kind of stability so its own forces can handle internal security.

Officials also acknowledge that the new operation was put together so hastily that planning has not been as thorough as the first time.

In effect, Reagan is putting U.S. prestige on the line in a second operation not as carefully circumscribed as the first one.

Nevertheless, the idea is to get the Israelis out of Beirut--by replacing them with the multinational force -- as a first step toward eventually removing all foreign military forces from Lebanon, part of Reagan's initiative. This would mean eventual withdrawal of Syrian and remaining PLO units, as well as Israeli forces, elsewhere in the country.

If things go well, however, the administration stands not only to rescue its peace plan but also to regain credibility in the eyes of Arab nations. They have reminded Washington that protection of Palestinians in Lebanon is essential because fear for the lives of those left behind was one of the main reasons that PLO fighters were reluctant to leave Beirut.

As the situation worsened in Lebanon in recent weeks, Reagan voiced some of the sharpest criticism of the Israeli government ever uttered publicly by an American president. Yesterday's speech continued that pattern.

For Reagan, who campaigned as a staunch supporter of Israel, the recent statements seem to be a dramatic turnabout with potentially grave implications for Israel and dramatizing the frustration of the Reagan White House with the Israeli government.

Yet officials close to the president believe that only Reagan's continued, visceral support for Israel as a nation, rather than for the Begin government, has kept the U.S. response from being even stronger.

As officials explain it, the president was horrified by the massacre in the refugee camps and deceived by a string of Israeli actions in recent weeks. He was persuaded by the assessment of his advisers that his peace plan "was going down the drain fast," as one official put it.