Israeli forces launched a ground invasion of Lebanon June 6, aiming to fulfill the Begin government's pledge to silence Palestinian guns once and for all.
The Palestine Liberation Organization (PLO), which operates primarily out of Lebanon, consequently was facing the most critical moment in its 17-year history. The Israeli Cabinet announced that the aim of what it called the ''Peace for Galilee'' operation was to take out Palestinian artillery that threatens northern Israel. But Israel's objective also could include pursuing the much-advertised goal of crushing the PLO.
After three days of escalating hostilities across the Israeli-Lebanese border and air raids throughout the southern half of this mountainous, seaside country, the Israeli invasion came at midday June 6 in a three-pronged initial thrust: up the coast toward the ancient port of Tyre, into the hilly central region near the Tibnin, and out of the Golan Heights toward the crusader-era Beaufort Castle. These were the primary Palestinian strongholds south of the Litani River.
Several hours after the invasion began, a United Nations official reported Israeli tanks had entered Tyre. Late in the day Israel landed soldiers as far north as Zaharani, just south of Sidon. The big Lebanese oil refinery there was reported to be in flames.
If the ultimate Israeli aim was, as Defense Minister Ariel Sharon reiterated last week, a military attempt to eliminate the PLO, then the Israeli Army would have to continue a massive, punishing attack deep into Lebanon. That, many observers fear, could draw in Syria (which has a 30,000-man troop presence in Lebanon), and other Arab countries.
The consequences could be far-reaching. Diplomats have heretofore predicted: a wave of radicalism throughout the Arab world; destabilization of moderate Arab regimes which refuse to fight; and perhaps some punitive constriction of America's oil supplies from this region of the world.
But Israel announced it would not engage Syria unless its troops interfered with the invasion. In Beirut, PLO leaders vowed to fight to the end.
At times June 6, the streets of Beirut fell almost silent as people listened to radios and traded the lastest news of the long-anticipated Israeli invasion. Other times, Palestinian gunners blasted in powder-blue skies at real or imagined Israeli jets.
On Beirut's outskirts, jeeps raced south covered with branches. The tail of an Israeli helicopter was paraded through the streets, but it seemed a small victory for the PLO in the face of the Israeli onslaught.
A leading Palestinian official put a brave face on the situation just before the invasion.
''We are tough,'' he said. Then he added: ''Besides, we have no place to go. Our backs are to the sea. If we were still in Jordan, they could push us into the desert. But we are here, and there's no place left but the sea - unless they expect us to go to Cyprus.''
This official from the Marxist Democratic Front for the Liberation of Palestine, said the developing Mideast crisis would be ''both good and bad for the PLO. It keeps our cause in front of the world. But people are dying.''
Among native Lebanese, feelings are mixed. Some sympathize with the Palestinian guerrillas, who since 1965 have been fighting to establish a homeland under the aegis of the PLO. Other Lebanese quietly say that if the Israelis crushed the PLO, this violent, war-torn country might be stabilized.
For Israel, the invasion has seemed inevitable. Israeli leaders and politicians have been saying the PLO must take responsibility for attacks on Israelis anywhere in the world. The June 3 shooting of Shlomo Argov, the Israeli ambassador to Britain, was seen as such an attack even though the assailants did not have a clear link with the PLO.
Prime Minister Menachem Begin, who is under stiff political challenge at home and rules only by the narrowest of Knesset margins, has promised Israelis living near the Lebanese border that they will not have to endure Palestinian shelling in the future. The shelling that has recently followed Israel's air raids into Lebanon, therefore, put Mr. Begin's word in jeopardy. Moreover, Mr. Begin knows from past experience that military action against the PLO plays well with the Israelis who reelected him last summer.
Two other important forces are driving the Israelis northward. One is water. The Litani River in Lebanon is much coveted by Israeli hydrologists and farmers to help feed Israel. Since Israel's 1978 invasion of southern Lebanon, there have been frequent Israeli attempts to tap the Litani. If Israel could secure the Litani, it might hand this water resource over to Israel's Lebanese ally, Maj. Saad Haddad, who might sell the water back to Israel. But to grab the Litani, Israel would have to move north and risk contact with Syrian forces in the Bekaa Valley.
The other force behind Israel's invasion is the biblical ''manifest destiny'' that is the raison d'etre of the Jewish state. Lebanon, according to many of Mr. Begin's supporters, was part of the ''promised land'' granted to ancient Israel. Certainly, the area through which Israeli forces were speeding June 6 will now come under--if not outright Israeli control--Israeli dominance in the future.