Second time around

This is the second, not the first, time that Israel has invaded southern Lebanon. There are some interesting similarities and some differences in the two stories.

The first similarity is the ease and quick success of the two operations. The first time, beginning on March 14, 1978, it took the Israelis 24 hours to take a six-mile deep bite out of southern Lebanon. They paused for two days. Then moved up to the Litani River in another two days of work. Opposition was ineffective.

The military story is the same this time. The Israeli forces moved at will as deep as they chose into southern Lebanon. The Palestinians were unable to put up effective resistance in either case. In military power the Israelis dominated the battle area then. They dominate it today.

There is an accompanying dissimilarity in the Israel purpose in the two operations. In March of 1978 they took everything up to the Litani River and seized bridges over the river. But they did not go beyond the river. Their obvious purpose was to clear the Palestinians out of the area between Israel's own frontier and the Litani River. They succeeded.

Ever since that 1978 invasion the area between Israel itself and the Litani River has been occupied in the lower part by the Christian Arab forces of Major Haddad. These forces are in Israeli pay and amount to auxiliary Israeli forces. Israeli troops have moved at will in and out of ''Haddadland.'' The upper part of this zone was occupied by a United Nations force.

This time the Israelis have set a 25-mile target, and had largely achieved it within the first 48 hours. It has taken them almost up to the southern outskirts of Beirut itself, and up against Syrian-controlled areas in central Lebanon. This is a much more ambitious operation with, presumably, longer range purposes.

In 1978 Washington joined most other countries in insisting on an Israeli withdrawal to be accompanied by a UN peacekeeping force. The sequel came quickly. The first phase of Israeli withdrawal began on April 11, less than a month after the initial invasion. Will there be a similar withdrawal this time, and if so how soon?

The biggest difference between the first and second Israeli invasions of Lebanon is in the framework within which the two military operations were launched.

In 1978 the Carter administration in Washington was negotiating with Saudi Arabia and Egypt for closer relations and for a program of US arms sales to both of them. Egypt was growing friendly to the US. But there had been no Camp David to commit Egypt to peace with Israel. That came in March of the following year. In 1978 Egypt could still renew hostilities with Israel, hence had leverage on Israel then which it lost by signing the peace treaty with Israel in 1979.

In 1978 the Shah was still in control in Iran. He was friendly to Israel, but he was also at peace with Iraq, his Arab neighbor. The Arab countries had their differences among themselves, but there was no major dissension among them. It was possible in 1978 for a more or less united Arab community to take collective action against Israel. Add that in 1978 Western Europe was decidedly pro-Arab and anti-Israel. Israel had little room for maneuver and enjoyed little tolerance in the outside world.

The difference in this respect is startling. Iran has been through a revolution, has sustained an invasion by Iraq, has repelled the invaders, and now is able to launch -- and probably is poised for -- a counterinvasion of Iraq. The success of Islamic fundamentalism in Iran is infecting the whole Middle East. Signs are detectable almost everywhere. The pro-Western and more modern Arab monarchies of the Gulf are uneasy, or worse. Iraq is probably due for at least a revolution against its now unpopular regime and might even be fragmented. Syria and Iraq are at swords' points.

Meanwhile Egypt is bound by a treaty with Israel to keep the peace, and in the US this is a mid-term election year. That means that the Republican administration in Washington would be particularly reluctant to offend Israel's many and influential friends in the US.

In other words, external circumstances which constrained Israel in 1978 have been replaced by new circumstances which give Israel greater room for maneuver than it has probably had at any time since it came into existence in 1948. Prime Minister Menachem Begin has a sharp sense of timing.

Israel Defense Minister Ariel Sharon has been urging this invasion for roughly a year. He has had his forces ready for it for roughly a year. Mr. Begin held him back repeatedly for a year. This time he let him go. For the short-term this is safe and a sure thing, an easy Israeli triumph, and a deeper security zone to the north.

Whether it will weaken or strengthen Israel in the long term is another matter.