Miscalculation of the military situation in the Middle East lies behind the collapse of Washington's effort to support the Gemayel regime in Lebanon. In November of last year the assumption was made at the White House that ''a strong demonstration of US support for Israel would convince Syria that it could not win support of Lebanon by military means.''
Accordingly Prime Minister Yitzhak Shamir of Israel was invited to the White House. Two days of conversations, on Nov. 28 and 29, were followed by the announcement of new arrangements for ''strategic cooperation'' between the United States and Israel.
White House officials defended the new arrangements on the ground that Syria would be impressed by closer US-Israeli ties and would become reasonable without withdrawing from Lebanon.
The opposite happened. Syria demanded that the Gemayel regime repudiate the May 17 Lebanon-Israel treaty. It supplied a military offensive against the armed forces of the Gemayel regime. That campaign reached its victorious climax last week in the disintegration of the official Army of Lebanon and the loss of most of the territory it had controlled.
The White House policy of closer military cooper-ation with Israel had produced the opposite of the desired and intended effect. It became a millstone around the neck of the Gemayel regime.
Behind these events was failure at the White House to realize that a new military situation exists in the Middle East. From the original Arab-Israel war until 1982 Israel possessed the most powerful military in the area. It dominated its neighborhood. It was able to take the offensive at will. It won all its wars.
The Israeli invasion of Lebanon in 1982 changed all that. It undermined Israel's economy. It eroded the popular will for war and divided the country. As of today Israel could still, undoubtedly, repel any attempted invasion. Its material military power is probably as high as ever.
But effective military power is made up of many elements. One vital element is a national will to war. In Israel today that will to war has been eroded. And the Syrians knew it.
The US landing on Grenada was a military operation Washington could not lose. It was carried out deep inside the military frontiers of the US. No unfriendly power could or would dare to intervene.
Lebanon does not lie deep inside US military frontiers. It is much closer to the Soviet Union, which has resupplied Syria with modern weapons and put some 7, 000 Soviet troops into Syria to stiffen Syrian forces.
Syria was able to support Muslim militias in Lebanon without fear of effective counteraction. It knew that neither Congress nor American public opinion would authorize an increase in the number of American troops or the offensive use of such troops.
Syria also knew that under present circumstances the government of Israel would not be able to launch an offensive war to sustain the Gemayel regime.
The Gemayel regime might, in theory, have been saved. Congress could, in theory, authorize sending half a million American soldiers to Lebanon and allow an attack on Syria. Such an attack could be twinned with an Israeli offensive against Syria. The outcome would then depend on how much support the Soviet Union would be prepared to give to Syria.
The outcome could become a general war between the US and the USSR. The US Congress nor the people of Israel would permit such a thing to happen. The threat of possible US-Israel military collaboration against Syria was a bluff. Mr. Assad knew, as did the Pentagon in Washington, that it would not work.
The collapse of Washington's plans for the Gemayel regime in Lebanon is like a barometer. It measures the climate. It discloses the fact that Soviet support for Syria and Syria's will to fight have altered the Middle East military balance. Israel has lost its offensive capability. The real question now is how much it can keep of what it has.