THE BEGINNING of the departure of the whole Beirut PLO apparatus from Lebanon represents in the first instance the freeing of a captive city from two months of terror. The PLO made the city an involuntary battleground; Israeli guns did most of the damage to it: a deadly "partnership." Its ending will give the long-suffering and immensely resourceful Lebanese the opportunity to start restoring the life that had made Beirut the most engaging and lively city in the Arab world. All those who are in a position to help the Lebanese in reconstruction -- not just in Beirut but in the countryside -- should turn promptly and generously to the task.

"Departure Day," as it is formally called in the plan ending the Beirut siege, is also a signal event in the life of Lebanon. It means, evidently, the beginning of the end of the hostile foreign occupation force that the PLO has been for nearly a decade. The removal of the PLO leaves two other foreign armies on Lebanese soil, Syria's and Israel's. Fortunately there seems to be a widespread determination to remove them, in time, as well. If that is done, Lebanon will be faced with the challenge -- running its own affairs -- that has proved exceptionally burdensome in the past. An early test of Lebanon's capacity for harnessing its disparate Christian and Moslem communities may come as soon as tomorrow, when the parliament is scheduled to select a new president. The one announced candidate, the Christian leader Bashir Gemayel, has in effect campaigned for Moslem support or at least tolerance by keeping at arm's length from his erstwhile Israeli patrons during Israel's most recent invasion.

The PLO could hardly have been expected to accept the ignominy of a forced departure and loss of its single military platform without seeking some political cover. This it has done by parading its endurance in battle and its intent to continue its struggle. To which responsible people can say: if you continue your struggle, it must be by political means--find a political course that is reasonable and realistic. Whether the PLO can hold together in the conditions of its new dispersion and speak effectively for its constituency and cause remains to be seen. So far, for instance, it has not even managed to address the question of the awful agony it inflicted on its Lebanese hosts. That should come even before the question of accepting Israel.

For Israel, in turn, must come the question of putting into effect the commitments to the Palestinians that it accepted at Camp David. There is a bruised, almost defiant tone to many Israeli pronouncements these days, in part perhaps a reaction to foreign criticism of its Lebanese operation. The fact is, however, that most other countries have endorsed at least implicitly some though not all of Israel's purposes in Lebanon: to secure Israel's border, to rout the PLO, to restore Lebanon. Israel is a lot less isolated than it may feel at the moment. Its friends, especially the United States, must point this out to Israel by way of obtaining its cooperation in the difficult diplomatic stages to follow. The decimation of the PLO as a fighting force has a special meaning here. The Begin government may be tempted to see it as opening the way to full Israeli absorption of the West Bank. Others will see it, correctly, as reducing the risk for Israel in walking the Camp David path further.

On the United States inevitably falls its own mission of leadership. One part of it has already been discharged with consummate skill by Philip Habib, who arranged the departure process that got under way yesterday. It is largely thanks to him that a role for American troops has been found that is at once useful, giving American diplomacy a place in subsequent phases, and limited, arousing minimal resistance at home. The French and Italians are also participating responsibly in the multinational force overseeing the PLO's evacuation from Beirut.

The president and his new secretary of state have been moving cautiously but steadily and saying the right things. Mr. Shultz has secured new commitments from Israel's and Syria's foreign ministers promising the withdrawal of their countries' armies from Lebanon. These pledges do not end all danger of an Israeli-Syrian partition or condominium but they point in the right direction. Mr. Reagan has comfirmed his intent to stay true to his predecessor's Camp David promise to try to resolve the Palestinan ussye "in all its aspects." This falls short of the immediate American endorsement of Palestinian "self-determination" that Egyptian President Mubarak urges on the opposite page today. Still, the important thing is that the United States, with its friends, sees the opportunities the Israeli invasion has created for treating in a careful, deliberate and unflinching wat the root cause of the Arab-Israeli dispute.