The Middle East is the home office of duplicity, of the double cross, of words that can simultaneously mean everything and nothing, with either one cause for men to kill and nations to go to war. Now, though, something new has entered the Middle East picture: paradox. The disunity of the Arabs has been replaced by the disunity of the Jews.

Given the history of the region, you can count on the situation to change. But for the moment, the Arab states -- with the exception of Libya -- have found common cause: Iran. They fear it and they do so with such intensity that Iran has replaced Israel as the pariah state of the region. Like the prospect of one's hanging, a genu-ine threat marvelously focuses the mind. Inthe Middle East, the ayatollah looms with anoose.

Maybe out of fear of Iran, maybe out of weariness and maybe out of maturation, the Arab states have collapsed into unity. Meeting in Jordan recently, the Arab League took some dramatic steps. It not only denounced Iran, it also endorsed a Middle East peace conference and authorized any Arab state to restore diplomatic relations with Egypt, the sole Arab nation to have made peace with Israel. In addition, the usual anti-Israel vitriol -- as much a fixture at these meetings as dark, sweet coffee -- was missing.

On the other side of the Jordan River, though, there is no such unity. Israel's ironically named unity government remains divided on the question of an international peace conference. The Labor Party and Shimon Peres want one. The Likud bloc and Yitzhak Shamir do not -- especially one in which the Soviet Union would participate. Unfortunately for both Peres and the cause of peace, Shamir is the prime minister. Among other things, he will not bargain away any of the West Bank in return for peace. To him, as to many Israelis, these are the ancient Hebrew lands of Judea and Samaria. On certain matters, Shamir takes instruction only from the Bible.

Consistency is to be admired, and Shamir is nothing if not consistent. But as he tenaciously clings to his position, the Arab world is in flux. Iraq, a rabid Israel hater, is fighting for its life against non-Arab Iran. Syria, the other hard-line regime, has economic miseries and is on the dole from more moderate Saudi Arabia. As for Syria's ally, the Soviet Union, it has said it will countenance no aggressive war against Israel. Even the Palestine Liberation Organization seems to have lost ground. The English version of the Arab League's final declaration failed to refer to it as the "sole legitimate representative of the Palestinian people." A fuming Yasser Arafat hit the roof.

Most important, the Arab states endorsed an international peace conference. This is the brainchild of King Hussein of Jordan, the first Arab leader to have resumed diplomatic relations with Egypt after the Camp David accords. Hussein was ebullient at the conclusion of the meeting. He had a right to be. Suddenly, moderation has swept the Arab world like a dry wind from the desert. Egypt, probably the most powerful of the Arab countries, but an outcast for making peace with Israel, is being welcomed back into the fold. When it comes to Iran, the Arabs can use all the muscle they can get.

Of course, nothing is simple in the Middle East. The Arab League's final declaration, while fairly brimming with moderation, insists on the "recovery of all the occupied Arab territories, including 'Al Quds Al Sharif,' " which happens to be Jerusalem. On Jerusalem, there is no division in Israel. It will not be returned to Arab control. It is the heart and soul of the nation -- the beacon that for ages the most distant Diaspora Jew could see clearly. Not an inch of it will be ceded.

The Arab language on Jerusalem might just be boilerplate, or it might be sincere. Either way, it's an issue for negotiators. But there won't be any of them if Shamir persists in missing what might be the best opportunity for a Middle East peace in years. He's a stubborn man, but he's also a national leader. He can be influenced, and no one is in a better position to do that than the United States, Israel's main benefactor -- not to mention protector. Now is the time for the Reagan administration to lean on Israel. Billions in aid should not buy servility, but it ought to buy something.

For years, Israel's foremost ally in the Middle East was the disunity of its Arab enemies. No matter how innocuous a peace proposal was, some Arab state would take offense and Israel could, with justification, say it tried. Now through the efforts of King Hussein, the Arabs have a modicum of unity, and it's Israel that's not trying. As a turn of events for the Middle East, this is more than a paradox. It's a tragedy.