President Reagan has just told a cheering British Parliament that "armed aggression must not be allowed to succeed." He was talking about the Falkland Islands, not Lebanon.

The Israeli army has, however, moved into Lebanon in great force. With its overwhelming strength as the world's third military power, it has pushed aside and destroyed with ease much of the PLO. Its announced goals is to push the PLO back 25 miles from the Lebanese-Israeli border and thus protect border communities from shelling. The Israeli forces have already raced far ahead of that 25-mile line. They are moving toward full war with Syria.

The border settlements in northern Israel are safe, but at an appalling and totally disproportionate cost. Southern Lebanon has been decimated, historic cities destroyed and thousands killed--Lebanese and Palestinians and, now, Syrians.

But that is not all. American presidents for decades have committed themselves to the territorial integrity, the unity and the political stability of Lebanon. That commitment appears faint today.

What does the future hold? The Palestinian cause will not go away because the PLO has been driven out of the south. No matter where the Israeli drive ends, there will be Palestinians, more bitter and more frustrated, just over the horizon.

Israeli leaders have said that they do not seek an inch of Lebanese territory, yet they add that a peace treaty has to be negotiated with Lebanon as a precondition to withdrawal. This strongly suggests prolonged occupation. There are no Israeli-controlled surrogates strong enough to govern all the territory that Israel has seized.

The U.N. has shown its incapacity. A U.S.- led force, as proposed by the Israeli envoy, Gen. Nathan Sharony, would find itself right in the middle of one of the world's most dangerous areas. It would not be the Sinai, where American forces are the guarantors of peace. In Lebanon they would be in a new role: Israel's policeman in another occupied area.

The partition of Lebanon may well be the eventual outcome. Israeli control may stretch to Beirut plus a Maronite-controlled state to the north of that. A Moslem state, doubtless housing Palestinian refugees from the south, would be centered on Tripoli in the far north, whle the Syrians remained in control of the Bekaa valley in the east. But this dismemberment will not bring peace to the Middle East or solve the Palestinian problem. Nor will it accord in any way with our historic support of Lebanon. The Christian community, including the heavily armed Phalange, must tremble at the prospect, despite its hatred of the Palestinaians and its cautious relationship with Israel.

There is another reading. Israel may well have decided to push the Syrians completely out of the Bekaa valley and to take out the Soviet-supplied missile batteries stationed there, as Prime Minister Menachem Begin has repeatedly warned Israel might do. Israeli aircraft are now carrying out such a mission. Syria, alone, could not withstand an Israeli onslaught.

Syria is allied with the Soviets. The Soviets have not committed themselves to defend Syrian forces in Lebanon. But they know they would suffer a major political defeat if Israeli forces push into Syria itself in order to smash that perceived threat once and for all.

President Elias Sarkis of Lebanon has said repeatedly that Lebanon is doomed to pay for the faults of Camp David. That negotiation-- which eliminated the Palestinian diaspora and the PLO from the peace process--has bottled up the Palestinian military forces in Lebanon and, as we see, has led to a major Israeli strike against them once an alleged casus belli was found. But Israeli military action, supported implicitly by the United States, will not subdue national aspirations. It will only deepen the desperation of the Palestinians and the frustrations of the Arab states.

Ambassador Habib's mission of patchwork has been overtaken by events. Israel is no longer dealing with terrorists; it is on the brink of war. The United States cannot simply repeat the past and try a piecemeal approach to the complex problems of the area. It will have to start thinking how to do many things together.

It must address the entire Palestinian situation, preserve Israeli-Egyptian peace, find security for Israel within stabilized but not expanding borders and build the strong long-term relationships with Arab states that the United States vitally needs.