NO SOONER had Ronald Reagan received Israel's foreign minister to urge restraint in Beirut than, with the minister still in Washington, Israeli tanks shot their way across the "Green Line" into the besieged western part of the city. It could be simply the minor "improvement" of a military position that Israeli officials suggest, or an early stage of the major assault that the administration, deeply concerned about civilian casualties, has pleaded with Israel to forgo.

Although the military point of the latest assault is not yet clear, the political point is. Israel insists that what is needed to bring a diplomatic ending to the PLO presence in Beirut and Lebanon is not military restraint but military pressure, and it feels that the expression of American reservations merely fortifies PLO hard-liners, undercuts Ambassador Habib and makes a military conclusion all the more likely. The word is being passed that Mr. Reagan is getting tough with the Israelis. By deeds and words alike, however, the Israelis betray a calculation that they can have their cake and eat it, too: decimate the PLO, notwithstanding the human costs, and enjoy continuing American support.

Perhaps the Israelis are right. Perhaps they will accomplish their objectives of destroying the PLO, restoring Lebanon's integrity, making peace with Lebanon and moving on to bring West Bank Palestinians and Jordan into a successful Camp David negotiation. Perhaps the United States and even some Europeans and Arabs will come to appreciate Israel's toughness and daring. Perhaps--here is the nub of it--it is inconsistent for the United States to look forward to the larger strategic fruits of the operation without tolerating Israeli tactics, harsh as they may be. The Israelis have that in mind when they brush off signs of American squeamishness and assert that on the goals of policy, the United States and Israel are agreed.

Except that being squeamish--accepting a moral standard, measuring ends against means--is an essential element of the American character and of American policy, in whatever administration. No argument of political convenience can alter that fact. That Mr. Reagan appears so eager to grasp a potential strategic opening that he cannot bring himself to protest Israel's assaults more effectively is not the end of the argument but its beginning. The question for Israel is not whether it can get away with its tactics but whether it should try to.

We are mindful of the assaults that others have made on Lebanon's hapless residents and of the element of self-denial that leaves a vastly superior Israeli force still parked on the outskirts of West Beirut six weeks after arriving on the scene. Still, we believe that Israel is storing up deep resentments among Americans and others as, of course, among the immediate victims and among those it regards as its future diplomatic partners. We are not convinced, moreover, that such severe tactics are essential to the resolution of the Beirut crisis by diplomatic means. Israel takes a tremendous risk by continuing to employ them.