The assassination of Park Chung Hee points up a fundamental infirmity in global politics. The world is full of countries where the death of a leader means a succession crisis with grave implications for the international balance of power.

In most of these places, including especially the Soviet Union, the United States has more than marginal influence. Thus, like it or not, this country is compelled to play leadership politics on a world scale.

President Park, of course, was unique in many respects. His rule of 18 years was long. He gathered into his hands control over economic development and the domestic political apparatus as well as external and internal security.

Difficulties in South Korea inevitably whet appetites in North Korea. Japan has to think again about its security arrangements. The presence of 38,000 American troops in South Korea, accordingly, is a crucial factor. It is worth recalling that in keeping with ideas fashionable only a few years ago, Jimmy Carter came to office with a promise to withdraw those forces.

In two other improtant Asian countries, leaders with as much authority now hold sway -- Suharto in Indonesia and Marcos in the Philippines. There is opposition to both and the likelihood of trouble when they leave the scene. American power, on the spot in the Philippines and in waters neighboring Indonesia, again represents a force for stabiltiy.

Anwar Sadat of Egypt is the leader on whom destiny has bet highest. Peace in the Middle East depends on his continued political and physical health. This country already shores him up with some economic and military assistance. But there is no substitute for Sadat -- which means that Washington ought to push ahead full steam, and without irrelevant diversions, with the Egyptian-Israeli peace treaty. The trick now is to make that process an irreversible reality before Sadat is obliged to quit office.

Three leaders in the communist world also cast figures much larger than life. Marshal Tito, at 87, is the senior figure among all world leaders. He claims to have made disposition for the succession in Yugoslavia. But his successors will be able to keep Yugoslavia outside the Soviet orbit only if the United States continues to accord them the support that was given Tito in his resistance to Moscow.

Deng Xiaoping of China is engaged in a race against time. At 75, he has only a few years in which to apply and secure for the future his program of economic modernization. When he goes, modernization will surely come under attack from orthodox Maoists -- linked perhaps with Moscow.

The United States, accordingly, has a strong vested interest in dealing cards to Deng. In that respect, the critical point -- the point that needs to be made over and over again -- is that the United States regards as a vital interest the security of China.

Finally, there is the case of Leonid Brezhnev. He is 72 and ailing. When he goes, a fight for the succession will take place -- either immediately or as soon as his aged colleagues in the collective leadership depart. No one can say how the succession struggle will end. but experience teaches that while the jostling is under way, the Soviet Union will not be able to advance far toward accommodation with the United States.

It happens that Brezhnev has tied his place in history to better relations with this country. He pushed through two treaties for arms limitation and has shown interest in thinning out forces in Europe and fostering more trade.

So if the United States cares aboutp those thngs, the present is a pregnant time. Rapid ratification of the arms control treaty now before the Senate will make it possible to move forward on the next steps-whether they involve further understandings on the other hand, if the arms control treaty is allowed to languish in the Senate, it and all of Brezhnev's other initiatives are apt to become playthings of the next Soviet leadership crisis.

Whatever happens, the central fact is that American foreign policy has to play leadership politics. Concentrations of power like that held by President Park in South Korea exist all over the world. This country can no more ignore their existence than it can wish away the oceans and the mountains.