Israel's blitzkrieg invasion, and the prospect that its final conditions for withdrawal likely would require a new political order in Lebanon, have caught both its local friends and foes off balance and plunged them into uncertainty.

For the Palestine Liberation Organization, the speed and extent of the Israeli advance--and the failure of the United States or other major powers to stop the violence--have heightened long-expressed fears that Israel is determined to smash its military capability for good.

The bulk of the PLO fighting force is thought still to be intact despite reported heavy losses in the south. But its rapid loss of territory has tended to belie Israel's oft-repeated claims, particularly those made during the past several months to justify an invasion, that the Palestinians constitute a regular army rather than just a guerrilla force.

Some diplomats and Lebanese and foreign analysts believe that the eventual outcome of the invasion could be the demise of the leadership of Yasser Arafat, who may be forced to make way for younger, more radical men ready to refute his moderate emphasis on political and diplomatic means to achieve the ends that never seemed within military grasp.

Clearly, Israeli Prime Minister Menachem Begin's declared objective of nearly doubling to 25 miles wide the current buffer between Israel and the area of Lebanon in which the Palestinians operate--and which the Israelis want to be policed by some international force--would change the physical face of Lebanon. But whether the achievement of those goals, even along with the long-term Israeli aim of forcing a withdrawal of the 23,000 Syrian troops here, would alter significantly the problematical political face of the country is another question.

"The straightforward military men and advisers around Begin apparently think they can kill a maximum number of guerrillas, impose their ideal government on Lebanon, send the remaining Palestinians back to refugee camps, or assimilate them, and truck as many possible north to Tripoli, as far away from Israel as possible," a Western diplomat said.

Yet if the past eight years of nearly nonstop violence have taught the Lebanese anything, it is that there is no easy formula to control either the Palestinians or the overall mosaic of 16 officially recognized minorities within the country.

Even Israel's erstwhile military friends here, the Christian Maronites, are having to rethink fundamental assumptions in light of the Jewish state's declared aim of restructuring this country.

Theoretically, the Maronite militias should be delighted with Begin's plans for Lebanon. The militias' leadership often has made similar calls in the past for the ousting of Syrian troops and the expanded presence of an international force.

Yet for public consumption at least, the days are gone when the Maronites of Bashir Gemayel's Lebanese Forces dreamed of just such an invasion that they could join in to liquidate the Palestinians militarily and bring them to heel politically.

Back in 1970 many Christians were delighted when King Hussein of Jordan smashed the PLO as a military force. But they learned to rue the day when the guerrilla remnants set up shop and became a state within a state here.

In both 1978 and 1981, the Christian militias deliberately provoked showdowns with the Syrians--who were not clever enough to avoid them--and then waited in vain for the much-hoped-for salvation to come from Israel.

Since the invasion, Gemayel and his principal lieutenants on his "war council" have been meeting almost continuously to examine events that have outpaced even the quick-witted Lebanese ability to predict.

"It's a great shock, and people do not realize what it means yet," a Gemayel aide said.

The militia leadership seems torn between fears that once again they will get burned and hopes of realizing their dream of outside intervention, which would return unquestioned political power in Lebanon to the Maronites.

Thus, diplomats and analysts nervously watch the progression of Israeli forces up the Mediterranean coastal highway toward Beirut.

Caught uncomfortably in the middle are the Syrians.

About 23,000 Syrian troops, formerly part of the Arab Deterrent Force called in to police the end of the Lebanese civil war in 1976, long ago pulled out of their former positions along the Mediterranean Coast. In recent days they have abandoned some of their checkpoints in Beirut to concentrate in the eastern Bekaa Valley adjoining Syria.

Were the Syrians to quit the capital entirely--or leave behind just the Syrian-officered regular troops of the Palestine Liberation Army who manned the so-called Green Line dividing the city--there are grave fears that the Christians might be tempted to invade predominantly Moslem West Beirut and join up with the Israelis.

A popular theory goes that the Christians, especially in tandem with the Israelis, could make short shrift militarily of the Palestinians and the detested hodgepodge of more than 40 militias that pretend to govern Beirut when they are not fighting among themselves.

With armed opposition thus eliminated, the fledgling Lebanese Army, which disintegrated during the 1975-76 civil war but now numbers 21,000 troops, could be sent south to police the territory the Israelis have conquered from the Palestinians.

Yet that scenario causes even more headaches here.

"If the Lebanese Army went south in those conditions," a diplomat said, "that would be considered worse in Arab eyes than another Egyptian separate peace with Israel."