Iran's invasion of Iraq, the latest twist in the Gulf war, is both threatening and complicating.
It threatens the stability of the entire Gulf region - a cause for concern to both the conservative Arab rulers on its shores and to the Western world so dependent on the area for its oil.
It complicates further the already complicated crisis in Lebanon. It diverts the attention of Saudi Arabia and other Arab moderates. They have a potential role in resolution of the Lebanon crisis but some of them feel more immediately threatened now by Ayatollah Khomeini's fundamentalism than by the military might of Israel encircling Beirut.
To that extent, for all Ayatollah Khomeini's anti-Zionist rhetoric, there is a net gain for Israel in the Iranian crossing of the Shatt al-Arab estuary into Iraq. Israel has, in fact, been a source of arms for Iran at moments of need since the Gulf war started 22 months ago.
Much in the days immediately ahead depends on the precise aims of the Iranians in carrying the war - launched by Iraqi strong man Saddam Hussein in September 1980 - back across the border into Iraq. An Iranian radio broadcast in Arabic July 14 said the purpose of the Iranian offensive was to topple Mr. Saddam.
There has been a personal vendetta between Ayatollah Khomeini and Mr. Saddam ever since the latter forced the Ayatollah in 1978 to leave the Iraqi Shia Muslim holy city of Najaf, where he had been living in exile for a decade and a half. From Najaf, the Ayatollah went to Paris - and the Western world became aware of him.
Ayatollah Khomeini is an uncompromising and vengeful man. He feels that he triumphantly humiliated first the late Shah, and then United States President Carter by forcing them from office. Since then, he has had Saddam Hussein at the top of his ''hit list.''
Any need for Iraq to pay $150 billion in reparation now is moot - as is any need to take back the tens of thousands of Shia Muslims whom Mr.Saddam expelled from Iraq into Iran. (Iran is predominantly Shia Muslim. Of Iraq's Arab, non-Kurdish population, Shia Muslims outnumber orthodox Sunni Muslims. Mr. Saddam is a secularized Sunni Muslim - two further counts against him in the Ayatollah's eyes.)
But the confrontation on today's battlefield between Iran and Iraq is not just personal or of recent origin. It goes back to an Arab victory over the Persians at Qadisiyah in that same area in 637 A.D.
That victory - invoked by Mr. Saddam when he sent his troops into Iran in September 1980 in the vain hope of toppling Ayatollah Khomeini - drew the rough line between Arab and Persian cultural and political influence that has persisted ever since. The line coincides broadly with the geographical division between Sunni Islam and the concentration of Shia Islam synonymous with Persian political power and national identity.
Shia Islam does spill over into Iraq, and paradoxically Shia Islam's two holiest sites are inside Iraq. These are the tombs of Ali and Hussein - who might in oversimplified terms be called the founders of Shia Islam - in the Iraqi cities of Najaf and Kerbala respectively.
The Iranians doubtlessly count on a measure of disaffection among Iraq's Shia Muslims against the Sunni Saddam Hussein in their thrust to unseat him.
The Iranians are in an aggressive and assertive mood, buttressed by the religious fervor that comes with the closing days of Ramadan, the holy Muslim month of fasting, which ends July 20.
They have after all turned the tables on Saddam Hussein, who had counted on quickly toppling Ayatollah Khomeini with his invasion of Iran 22 months ago. Now it is they threatening Mr. Saddam from a foothold on Iraqi territory - and causing anxiety among all the other Arab states of the Gulf.
Iranian self-confidence has shown itself defiantly in recent days - first by defying an Arab-led call at last week's OPEC meeting for ceilings on individual oil production by the main producers, and second by ignoring a unanimous UN Security Council call July 12 for a cease-fire in the Gulf war.
It remains to be seen how far the Iranian Army will go into Iraq - and whether Iraqi soldiers will do better in defending Iraq than they did in invading Iran.
The Iranians might try to take or at least besiege Basra, Iraq's only major outlet on the Gulf. They might content themselves with cutting the overland route from Basra northward to Baghdad - vital for getting supplies into the interior of Iraq from other Arab Gulf states. (Basra itself has been immobilized as a port of entry for many months.)
Syria - the only Mideast Arab state to support Iran - closed its frontier to with Iraq earlier this year. That leaves open only the routes to and from Iraq by way of Jordan or Turkey. In theory then, Iran could rely on economic strangulation rather than a deep military incursion to achieve its aims in Iraq.