The Israeli government and those American apologists who marched lockstep with it into Lebanon had trouble enough making their case before the killing of President-elect Bashir Gemayel. And what now, after last weekend's slaughter of innocent Palestinian refugees at the hands of rampaging Christian militiamen whose very existence, if not their very act, owes much to Israeli support?

Not even by the most brazen dissembling can the case be made today. The sweep into West Beirut in the name of "stability" (and in violation of plain undertakings to the contrary), and its ghastly consequences, have laid bare the bankruptcy of Israeli policy. The more hollow the achievements of the Begin government are proved to be, the more it must seek to achieve, in redemption of what it could not achieve. And so it goes on, in endless, escalatory pursuit of a sense of security that is unachievable by single- minded brute force.

The point is made most forcefully when Israeli "gains" are measured against Israeli claims. Consider, in composite, the argument set forth in recent days by Prime Minister Begin, Defense Minister Sharon and Foreign Minister Shamir. It asserts: the destruction of the PLO as a military force; the creation of a bright, new opportunity for Lebanon to regain its sovereignty; the smashing of the "infrastructure" of international terrorism; the prospect of an early peace between Lebanon and Israel; freedom for "moderate" Palestinians to participate in the autonomy process on the West Bank and in Gaza without fear of PLO retaliation.

"The expulsion of the PLO means that international terrorism has been dealt a mortal blow," Sharon wrote recently in The New York Times. "The whole infrastructure of violence and revolution has been broken." So how come the Israeli government is loudly protesting the continuing presence of PLO pockets of resistance in Lebanon as justification for Israel's continuing occupation? As for the crushing of PLO terrorism, the most significant single "mortal blow" of late was the killing of Gemayel -- an act of terrorism that the Israelis instantly credited to the PLO.

Now that the PLO "terrorists" are gone, wrote Sharon, "I believe Palestinians will come forward prepared to negotiate with Israel on the autonomy plan proposed by (Begin)." No doubt -- if Sharon means Israel's hand-picked Palestinian collaborators on the West Bank. But if he is talking about the autonomy formula agreed to by Israel at Camp David -- the one that bears Egypt's signature as well, and requires Jordan's participation -- its prospects are further dimmed, as if by calculation, with every new Israeli affront to Arab moderates.

With the PLO now isolated and enfeebled, Shamir said the other day, "it will very soon disappear from the political stage. Who will pay attention to their speeches if these are not accompanied by atrocities and massacres?"

Pope John Paul II gave him a quick answer. So, indeed, did Arabs of all stripes at their Fez summit meeting. So have the Europeans. But that is not the worst of Shamir's astonishing claim. What he is suggesting is that the PLO, and by extension its vast legion of Palestinian supporters, cannot get attention except by terrorist acts; they are damned if they do and doomed if they don't. By indirection, the Shamir argument would place a premium on PLO terrorism.

So much for the "gains." Now consider some other consequences of Israel's new imperialism -- the ones that Begin & Co. don't talk about. Ronald Reagan has a Middle East policy, updated from, let's say, the 1950s to at least the late 1970s. It is Camp David in its truest sense -- with a role for Jordan. Begin hates it. But it was Begin's crude contempt for Ronald Reagan that created it.

Begin has only himself to blame, as well, for the splintering of what once was automatic, reflexive support among the American Jews for Israeli policy. By what he did to energize the Reagan administration, Begin has also given his own political opposition something to fasten on to as an alternative to the Begin theology that would lay claim for eternity to the West Bank as an integral part of Israel. Abba Eban, the former foreign minister and the present foreign policy spokesman for the Labor Party, has pronounced the Reagan plan to be "a useful basis for dialogue and analysis."

By no test, then, can Begin rate his Lebanese adventure a success. The minuses are his to justify as best he can. But the pluses are Reagan's to build upon. And this can only be done by finding some way to bring American influence to bear constructively on Israel.