It has been my observation, and a keen one at that, that you cannot judge a relationship by the words used to describe it. Thus, a couple that calls one another "darling" and "princess," "honey" and "sweetheart," may be saying these things more out of habit than anything else. The relationship has ended, but the vocabulary lingers on.

This may be the case with Israel and the United States. In the last few months, this odd couple has sharply disagreed over President Reagan's peace plan, has not been able to agree on a formula for sharing military information and has played chicken with one another in Lebanon.

Through it all, though, the word "allies" or a variation thereof continues to be used. It surfaced recently in the letter Marine Commandant Gen. Robert H. Barrow sent to Defense Secretary Caspar Weinberger in which he complained of the way Israeli soldiers allegedly have been treating his Marines: "It is inconceivable to me why Americans--serving in peace-keeping roles--must be harassed, endangered and degraded by an ally."

Something is going on here. Barrow's letter was just the latest squawk from the Pentagon about Israeli military manners. Earlier, there was the by-now famous incident in which Marine Capt. Charles B. Johnson, armed with nothing but a pistol and the mistaken notion that he was John Wayne and Israel was the enemy, stared down three Israeli tanks. Hold your fire, Johnson. We're not at war with Israel yet.

But there have been other incidents, as well. Some of them have involved the Marines and some the U.N. forces to which American troops are attached. As usual, the Americans blame the Israelis and the Israelis either blame the Americans or claim that they don't know what the U.S. is talking about.

The upshot, though, is that the Israelis and the Pentagon--these two allies, these old buddies of multiple-exchange programs and mutual antipathy toward the Soviets--are now trading charges and denials. We do not even know for sure if American troops have been forbidden to fraternize with Israeli soldiers. The Pentagon says no; the Israelis say yes, but the troops, apparently, say nothing to each other.

Whatever the reason for these disputes, it's apparent that for the moment Israel and the United States are allies in name only. Aside from a common interest in keeping the Soviet Union out of the Middle East and a general desire for peace (whatever that is and whatever the terms may be), the two can't seem to agree on anything else--not the terms for a truce in Lebanon, not the terms for peace in the area and not even, it seems, who has the right-of-way on Beirut highways. Some alliance!

To an extent, these are short-run problems and maybe nothing to worry about. The trouble is that these differences are occurring at the very time when both the U.S. and Israel have troops in the same area. It's an unhealthy mix. Israel will never be satisfied with American diligence when it comes to dealing with terrorists. And Americans are not likely to understand Israeli zeal when it comes to hunting down terrorists. It is a job Israelis are loath to leave to others, thinking they are not likely to bring to it the proper degree of enthusiasm.

There is a lesson in all this that somehow gets overlooked in all the easy talk about stationing more American troops in Lebanon--especially in the south. There, the risk of terrorist infiltration is much greater and so, as a consequence, is the chance for American-Israeli confrontations. In that event, American-Israeli policy will be influenced, if not set, by military men either chasing terrorists or inadvertently protecting them.

It would be an overstatement to say that the special American-Israeli relationship is over. It's not. But words like "allies" and terms like "mutual interest" mask some profound differences. For that reason, if not for that reason only, the United States, of all countries, should not provide troops to help secure the Israel-Lebanon border. At the moment, the U.S.-Israel relationship is mostly a matter of words. An exchange of fire, though, and it could be on the rocks.