Jerusalem — Just as Israel was pushing ahead with its peace treaty commitment to return Sinai to Egypt, its war planes April 21 struck deep into Lebanon against ''terrorist targets.''
It is as though the deep emotions which have built up over the sacrifice of Sinai for the sake of peace on this country's southern border are being released northward.
Certainly, an important factor shaping Israeli policy is the fear of jeopardizing its security by making concessions which might give an appearance of weakness. Few observers here believed that the Israelis would let the Sinai withdrawal go through without some show of strength to make the point that this was not a retreat.
In addition, the US-sponsored peace treaty with Egypt has freed the powerful Israeli armed forces to concentrate on Israel's perceived enemies to the north -- especially the Palestine Liberation Organization (PLO) and the Syrians. In Wednesday's strikes, three Palestinian targets were bombed and two Syrian MIG-23 fighters sent to intercept the Israeli planes were shot down.
Even so, this was not the major ground assault on southern Lebanon which many analysts had long expected -- and for which Israeli ground forces massed along the northern border have been assumed to be poised.
One well-placed United Nations source suggests that the international uproar created by the April 11 shooting incident at Jerusalem's Temple Mount may have put off Israeli timing. (The shooting spree by an American-born Israeli reservist, which killed two Arabs, set off a chain reaction of protest throughout the Muslim world and beyond.)
Whatever the reason, and despite the country's current agitated state, the government opted for planes not tanks. And the latest ''blatant provocation'' -- the killing of an Israeli soldier in southern Lebanon by a land mine and other recent attacks on Israelis at home and abroad - triggered bombs rather than a ground assault.
Now that the opening round has been fired, however, analysts here say the PLO almost surely will feel obliged to retaliate -- giving Israel even more reason to launch a ground invasion. Israeli settlements along the northern border have been placed on alert.
Israeli chief of staff Rafael Eitan insisted April 21 that the air raid did not necessarily end the cease-fire which went into effect July 24, 1981. He said he hoped the Palestinians would return to abiding by the cease-fire. And the United States called on all parties to ''respect scrupulously the spirit as well as the terms of the cease-fire, which is of such importance to the stability and welfare of the peoples of the entire region.''
Meanwhile, the Israeli Army was occupied with a very different and distasteful task in Sinai. And as it removed the remaining Israeli pullout-protesters from their redoubts and turned the reclaimed desert back into sand dunes, the Begin Cabinet April 21 announced its unanimous determination to press ahead with the evacuation before the April 25 deadline set by Camp David.
''When I heard about Yamit being sewn into the desert, it just really hit me hard,'' said an Israeli reporter who has covered the Yamit drama. ''Do you know what it is like to think of that big, beautiful city disappearing?''
Indeed, as the agreements under the first phase of the Camp David treaty near realization, reactions in the region have become much more sharply polarized.
Press a conservative Israeli on the subject and he will respond: ''Peace treaty? How long can we expect Egypt to stay at peace after next week?''
A Palestinian under Israeli rule will say: ''Fine, Egypt has its territory. Now what about us?''
From the beginning, not everyone has been elated with the Camp David peace treaty. But, despite changes of government in two of the three signatory countries, the treaty is nonetheless alive. Egypt and Israel today stand on the verge of completing a three-year period of peaceful adjustment unprecedented in Arab-Israeli affairs:
* The 30-year state of war between Israel and its largest Arab opponent is over.
* The two countries have established diplomatic and trade links.
* The Egyptian-Israeli border is being returned to its pre-1967-war outlines.
What apparently convinced Prime Minister Menachem Begin that the Israeli withdrawal from Sinai should proceed were two letters he received April 20.
One, from Egyptian President Hosni Mubarak, pledged that Egypt intends to continue negotiations with Israel toward Palestinian ''autonomy'' along the lines laid down in Camp David. The other, from President Reagan, was described by Israeli leaders as ''of great significance.'' It apparently reassured Israel that the US was committed to Camp David-style autonomy as well as to Israel's security.
American diplomats have told the Monitor they expect Washington to make a major push in the coming months to reach an agreement on a ''self-governing authority'' for the West Bank and Gaza Strip. But these and European diplomats admit there is not much hope for such an agreement if: (1) only Egypt and Israel are involved, (2) Egypt is under no pressure to bargain, and (3) Israel is bent on annexing the territory.
''The problem is that Egypt sees a Palestinian state at the end of the road, '' one diplomat said. ''and Israel sees annexation of 'Judea and Samaria' into Eretz Israel ('the land of Israel').''
Few people in the Middle East, it seems, are going to take time out to celebrate the handover of the Sinai.