What Mr. Reagan knew

A week ago President Reagan held a press conference at which questions about the Israeli invasion of Lebanon were inevitable. He was primed in one respect for those questions. In his first attempt to answer he said:

''Well, you've asked a question, or several questions, that I have to walk a very narrow line in answering.''

But he could not have been briefed adequately on the subject because he proceeded in three separate passages during the press conference to make statements or observations most of which were wildly contrary to the known and well-reported facts.

First there is the question of advance knowledge of Israel's intentions. The President said in his first answer:''We were not warned or notified of the invasion that was going to take place.'' Later he said, ''We were caught as much by surprise as anyone.''

The invasion began on June 6. Here is a partial record of the warnings which had reached Washington in advance.

Feb. 9. The New York Times reported from Israel that Israel had drawn up plans for a large-scale invasion of Lebanon and had nearly launched it the week before. The report had been cleared by the Israeli censor.

Feb. 18. Israeli Defense Minister Ariel Sharon stated that Israeli armed forces had been on ''high alert'' since the previous December.

Feb. 25. Israel's new ambassador to the US, Moshe Arens, stated to reporters in Washington that an Israeli invasion of Lebanon in response to ''some provocative action by the PLO'' was increasingly likely. ''I would almost say it is a matter of time,'' he said.

Feb. 26. US special Mideast envoy, Philip Habib, returned to the Mideast to try to save the cease-fire.

April 9.The State Department in Washington officially noted Israeli troop movements near the Lebanon border over the previous three days and issued a statement saying, ''We renew our appeal to all those involved or with influence on those involved to show the utmost restraint.''

April 11. Israeli air force planes overflew Beirut and drew anti-aircraft fire. (Israeli spokesmen had asserted PLO forces near the Israeli frontier. UN truce observer forces in southern Lebanan denied these assertions.)

April 21. One Israeli soldier was killed by a land mine that exploded in southern Lebanon, in the area known as Haddad-land. The area is controlled by Arab forces in Israeli pay. Within hours of the incident waves of Israeli planes bombed PLO areas along the Lebanese coast for two hours in early afternoon in retaliation. This was the first substantial breach in the cease-fire, which at that time had been in force for nine months. The US deplored ''all cease-fire violations,'' including ''violence against the Israelis.'' The PLO did not retaliate.

May 9. Israeli planes bombed coastal Lebanon. Arab casualties were listed as 16 killed, 56 wounded. The PLO retaliated with ''about 100'' shells or rockets into northern Israel. There were no Israeli casualties.

June 3. Israel's ambassador in London, Shlomo Argov, was severely wounded when leaving the Dorchester Hotel. Israel officials immediately blamed the PLO. British Prime Minister Thatcher said that a ''hit list'' found on one of the suspects listed names of PLO officials as well.

June 4. The Israeli air force delivered ''massive strikes'' against PLO forces in Lebanon.

June 6. The Israeli invasion by ground forces began.

Then there is the question of provocation. President Reagan in his press conference said that the PLO had ''pursued aggression themselves across a border , by way of rocket firing and artillery barrages.'' Later he said, ''You must recall that prior to this attack Soviet-built rockets and 180mm cannon were shelling villages across the border in Israel and causing civilian casualties.''

The only rocket or artillery fire from PLO forces into Israel during the 11 -month truce previous to the attack was on May 9 in retaliation for the second of two Israeli bombing runs over coastal Lebanon in which 86 persons died.

The President's press conference was on June 30. On July 4 there appeared in the New York Times a half-page advertisement signed by 66 of America's most prominent Jews, including 7 rabbis, which called on all American Jews to ''speak out critically about those Israeli policies which we know to be mistaken, self-defeating, and contrary to the original Zionist vision.''

The President at his press conference denied that the US had given Israel a ''green light'' for the invasion of Lebanon. It was within his power to take action which could have prevented the invasion. He did not take such action. He says he was taken by surprise and he says there was provocation. The failure and the factually mistaken explanation for the failure are taken by most others as having been a ''green light.''