The most tragic thing about Israel's invasion of Lebanon has of course been the loss of human life, the human suffering, and the destruction of human property. In a different category the most disturbing thing about it is the extent to which this has been due to just plain incompetence in the handling of foreign affairs in Washington.
In a previous column in this space on July 8 I listed the many reasons why President Reagan should have known that Israel was preparing the invasion of Lebanon. The President claimed to have been taken by surprise and said no one had warned him. The record is clear that he could have been surprised only if his staff had failed to tell him what everyone else who follows world affairs knew.
The essential fact before the invasion occurred is that Israel had mobilized and deployed the forces for the invasion not only weeks in advance, but openly. There was no secret about it. The preparations were widely reported. Israel's ambassador in Washington even stated that the invasion was coming. The State Department issued warnings and pleas for ''restraint.'' But the White House failed to give the Israeli government reason to think that the President would disapprove if and when it happened.
Since then we have had further and even more disturbing evidence of the inability of the present White House to foresee the consequences of its own words, and lack of words.
The President received Prime Minister Begin at the White House on June 21. By that time the invasion had surged through southern Lebanon and up to the borders of West Beirut. Israeli air, land, and sea forces had been pounding away at that city for a week. The heaviest shelling to that time was on the day the President received Mr. Begin.
The President greeted Mr. Begin cordially. If the President expressed displeasure over the bombing and shelling of West Beirut he did it privately and in such muted terms that Mr. Begin showed no signs of having been restrained or warned. When the guest had departed the President told reporters:
''It's clear that we and Israel seek an end to violence there and a sovereign , independent Lebanon under the authority of a strong central government. . . . We agree that Israel must not be subjected to violence from the north.''
The heaviest bombardment of West Beirut came on Aug. 1. The President finally spoke out strongly. He said he had ''lost patience long ago.'' He said ''the bloodshed must be stopped.'' On the next day, Aug. 2, the President received Israel's foreign minister, Yitzhak Shamir, at the White House. Mr. Shamir was treated coolly. He was told that ''the world can no longer accept a situation of constantly escalating violence.'' The President wanted a ''complete end by all parties to the hostilities around Beirut.'' But two days later, on Aug. 4, Israel launched another powerful advance into West Beirut during which sections of the city hitherto spared were heavily bombarded and heavily damaged.
That attack expressed either deliberate contempt for the US President or a failure in the government offices in Israel to realize that Mr. Reagan really did want them to stop their assault on West Beirut. And that was the point when the President finally sent a strong letter to Mr. Begin saying in unmistakable terms that he did want the blooodletting to stop.
That presidential letter did seem to make an impression in Tel Aviv. The shelling did not entirely cease, but it did taper off. There has not been another major Israeli advance or attack.
Why did it take so long for Mr. Begin to understand that the President wanted him to stop his attack on West Beirut?
Early in the Reagan administration the President's security advisor, Richard Allen, identified Israeli forays into Lebanon as being justified under the doctrine of ''hot pursuit.'' Never during the Reagan administration has the White House protested the planting of more Jewish settlements on the West Bank. Reagan called them ''legal.'' The White House did not protest against the dismissal of Arab mayors of Arab cities in the West Bank and Gaza. The Israeli declaration that the PLO ''must leave Lebanon'' has been echoed by the White House and its validity accepted there.
To any diplomat the record means a clear ''green light'' for Israel to do what it has done. Any professional diplomat feels that Mr. Begin has a legitimate right to feel that he is now being told to stop doing what he must have thought he was authorized to do.
It is impossible in foreign policy operations to expect a client and dependent state to refrain from what it wants to do and prepares openly to do if its sponsor state, which provides the weapons, fails to tell it clearly to call off the operation and, on the contrary, accepts as valid the declared goal of the operation - to expel the PLO from Lebanon.