No beachhead for diplomacy

Three different world situations have been changed significantly by events of the past week.

In the Falkland Islands war the British have made good a large and secure beachhead and now enjoy a favorable military advantage over their Argentine opponents. Defeat of the Argentine invasion force in the islands seems likely in the near future.

In the Middle East the tide of war has turned even more decisively in favor of Iran over Iraq. The Iraqis have been driven from Khorramshahr. In effect the Iranians have regained all that they lost in the original Iraq offensive which began on Sept. 4, 1980. The fanatic figure of the Ayatollah Khomeini seems to dominate the Middle East. It is a triumph for him. Will his Islamic fundamentalism now spread and unsettle the whole area?

In the Far East, delicately, Chinese diplomacy has readjusted China's attitude toward the two superpowers. Its leaders have held quiet talks with Moscow's top China expert while their propaganda gives equally unfavorable treatment to both Soviets and Americans. China labels both ''imperialist hegemonists'' and increases its public emphasis on cultivating ''third world'' countries.

In the long run the change in Chinese position may be the most important of the three adjustments. The new China was the ally of the Soviet Union from 1950 to 1960. It broke out of that relationship in 1960 and sought a reopening of relations with the US. That change came about in 1972 when Richard Nixon became the first US President to visit Peking.

From 1972 to 1982, China used the US as a protection against military threats from the Soviet Union. Between a quarter and a third of the Soviet Union's military power has been deployed along the Chinese frontier during this span of time. Many a border skirmish has occurred. The US was treated as less dangerous to the world than the Soviets.

Now the Chinese are once more adjusting the pattern. They are keeping the Soviets at arms length, but also putting the Americans in a more remote position. Western diplomats in Peking talk of China having ''rediscovered the third world.'' China has so positioned itself now that it can play Moscow off against Washington or vice versa as best suits Peking's interest at any particular moment.

Continued sale of US weapons to Taiwan was the reason the Chinese cited for doing what they probably intended doing anyway; i.e. to cool their relations with Washington. They used their alliance with Moscow to protect them from US hostility during their first 10 years. They used the US to protect them from Soviet hostility over the past 10 years. Now they allow both superpowers to maintain embassies in Peking. They talk to both, coolly. But now that they are strong enough and their government sufficiently established - they can afford the luxury of independence from both.

Most unsettling to Western foreign offices of the three changes is the decisive military victory of the Iranians over the Iraqis. One measure is that Iraq, which has boycotted Egypt since Egypt's Camp David peace talks with Israel began, has said it would welcome Egyptian troops to help hold its frontiers against a possible invasion from Iran.

Will the Ayatollah, flushed with his victory on his own soil, now seek to punish Iraq by crossing the old frontiers?

Few Western experts would dare to predict the Ayatol-lah's future course of action. Some of them are inclined to think that he is more likely to attempt to spread his influence through his Shia Muslim faith than through the work of his soldiers. Iran is the leading and central feature in the Shia branch of Islam.

There are minority Shia groups in several other Muslim countries. South Lebanon is an example. The dominant Sunni Muslims tend to regard Shiaism as heretical, but in 1929 the equivalent of a concordat was arranged between the two. Its effect has been similar to the change which the second Vatican Council brought about in the Christian world between Roman Catholics and Protestants. Each accepted the legitimacy of the other's sacraments.

Ecumenicalism now prevails between Shia and Sunni Muslims just as it does between Catholics and Protestants. But whether it will continue is another matter. If the Ayatollah now uses the Shia communities as political fifth columns for his purposes - a lot may change. No matter what course the Ayatollah chooses to take, the mere fact of the spectacular military recovery Iran has made will be unsettling.

The Arab oil states on the west side of the Gulf are all uneasy. All of them are conservative, modern and friendly to their Western customers. All are threatened by the Ayatollah's version of Islamic fundamentalism. One, Bahrain, has recently suffered from an attempted rebellion. Kuwait, at the top of the Gulf, is literally within range now of the Ayatollah's guns. The others are Saudi Arabia, Qatar, the United Arab Emirates, and Oman. All are in danger of being unsettled by Iran's victory and the Ayatollah's future purposes.

In the Falkland Islands, British military professionalism and superior modern weaponry have produced the expectable results. The Argentines could inflict damage on the British task force. But their maximum efforts could not prevent the British from landing a force of some 3,000 highly skilled commando and paratroop units on the same main island on which the Argentines put their main invasion force. And the British now control the sea and air approaches to the Argentine enclave at and near Port Stanley.

It would seem to be just a matter of a few weeks or perhaps even a few days before the position of that main Argentine force is made untenable. Argentine conscripts are a poor opponent for the highly skilled volunteers of the British forces. This war seems likely to turn into an argument in favor of the all-volunteer force. One can usually get a higher level of military professionalism out of volunteers.