No green light

It is good to have on the record President Reagan's public statement that the United States did not approve Israel's invasion of Lebanon or give Israel a green light to send its troops into west Beirut. The American people cannot but be discomfited by the thought of even indirect US support for Israeli actions in Lebanon and complicity in the terrible devastation and civilian casualties caused by them.

Their dismay will be all the more acute in light of the President's comment that he ''believed there could have been a diplomatic settlement in that situation.'' Diplomacy, in other words, was cast aside in favor of all-out violence.

If that is so, the administration's conspicuous lack of criticism of Israeli policy seems oddly contradictory. Mr. Reagan understandably is careful not to roil the waters while a way out of the Lebanese crisis is being sought. But it may be the very reluctance of the United States to put starch in its stance toward Israel that has in effect given Menachem Begin a green light to pursue his own headstrong course. Presidential hints that Israel's use of US-supplied cluster bombs in Lebanon may be found to be ''defensive'' and therefore legal is but the latest sign of a catering to the Israelis. Yet, it may be asked, how long will Americans tolerate supplying virtually unlimited military aid to Israel when Israeli policy so often is at variance with the US national interest?

This is a moment for reassessment in Washington. With a new US secretary of state, it clearly will take time to sort out options and redefine American policy. But it is plain that, if the back of the PLO is broken, the issue of Palestinian nationalism is not. In his press conference Mr. Reagan cited three US objectives in Lebanon: reestablishment of a strong central government in Beirut with only one military force; a guarantee of the southern border with Israel; and the removal of all forces - Syrians, Israelis, and PLO - from the country.

Many would have felt reassured if he had added another: establishment of a self-governing homeland for the dispossessed Palestinian people. Only through vigorous commitment to this goal as well as to the security of Israel can lasting peace be brought to the Middle East.