The two superpowers - the United States and the Soviet Union - begin this weekend their second groping toward a formula for coexistence. They do so at a time when both have experienced severe limitations on the range of their powers and both find themselves in economic difficulties.
Their immediate goal is to cut the cost of the arms race, which is becoming too expensive for both.
They may find themselves pushing on from there toward other future arrangements that will further reduce their spending on guns by reducing the sense of mutual menace in their relations.
The first such groping dated from 1972, when Richard Nixon and Leonid Brezhnev thought they could stitch together a comprehensive package that would make them partners in world management. That one, called d'etente, collapsed during the Brezhnev years over Soviet economic egotism.
The Soviets of those years thought they were going ahead fast enough economically to be able to outbuild the US in weapons and hence could afford to expand their empire. Under Brezhnev, they invaded Afghanistan.
Today's world is a very different place from the world of 1972 to 1979.
Moscow's economic egotism of the Brezhnev years has been superseded by the knowledge that the Soviet Union has been stagnating and is now an economically backward country.
The men in the Kremlin know two things today that they did not know in 1979 when they invaded Afghanistan. They know that capitalism is not destroying itself, as they were taught in their youth to believe it would. And they know that their own form of communism means economic stagnation and must be changed if their country is ever to get back into the front rank of modernization.
Mikhail Gorbachev comes to Washington goaded by the horrifying (to him) probability that unless the Soviet Union goes modern, it could find itself outdistanced in wealth and power by China.
And Ronald Reagan meets him knowing that this is his last chance to convert a US arms buildup into a cut in Soviet weapons. If Reagan fails this time, it could be his successor who would reap the fruitage of his investment in military power.
The essential truth behind this Reagan-Gorbachev meeting is that the world positions of both the Soviet Union and the US have passed their zeniths and are on the decline.
The two emerged from World War II as superpowers. The liquidation by war and its aftermath of all the older empires had left the world wide open for the two. They began competing almost at once to fill up the empty spaces as the German, Japanese, and Italian empires collapsed in defeat, and the British, French, and Portuguese empires liquidated themselves.
The new Soviet empire reached its zenith in 1950 when China entered into a military alliance with Moscow, and the men of the Kremlin came into control of the enormous spaces that lie between the River Elbe in Europe and the China Sea.
Yet it was a short-lived empire of power. Yugoslavia had already broken out from under Kremlin discipline in 1948. China pulled out in 1958. Egypt had entered the Soviet sphere in 1956, but pulled away in 1972.
Mikhail Gorbachev inherited from his predecessors a much-reduced empire of power. He can still dominate his immediate neighbors in Eastern Europe, but he has only three important holdings beyond his own frontiers. They are Cuba, Vietnam, and Mongolia.
These three are Moscow's ``special friends.'' They receive 80 percent of Moscow's economic foreign aid. They are cultivated, sustained, and protected by Moscow.
Beyond the favored three are six other clients that receive some help, usually of modest dimensions and of a more or less tentative nature. They are Afghanistan and South Yemen in Asia; Angola, Mozambique, and Ethiopia in Africa; and Nicaragua in Central America.
Add to the list Syria and Iraq as Arab countries that are heavily armed by the Soviets but not ideologically or economically part of the Soviet system. They are temporary prot'eg'es rather than permanent clients or satellites.
Moscow treats its clients, other than the three ``special friends,'' as being expendable.
It has, for example, given Nicaragua enough weapons and fuel to keep the ruling Sandinista movement alive, but just barely. It has sent it enough helicopters to contain the contra rebels in the jungle, but has refused to send the requested fighter planes that would have given offensive power to the Sandinistas. It has provided enough oil and gasoline to keep the military effort alive, but always less than requested, and not enough to keep the economy comfortable.
The range of American influence over this same post-World War II era has also been restrained, first by partial military failure in Korea (the retreat from the Yalu), and then by total military failure in Vietnam.
The American ``empire'' probably reached its own peak in 1965, as then President Johnson began to feed a half million American soldiers into the jungles of Southeast Asia (against the advice of his best generals, who told him to avoid a land war on the mainland of Asia).
By Ronald Reagan's time, the American economy had also begun to lag. He had inherited a still dominant economy. It was still earning more than it bought when Mr. Reagan entered the White House. But his tax-cutting policy stimulated American consumption beyond American production. The balance of trade turned against the US. It funded its deficits on borrowed money.
By now, the willingness of friends and partners has been exhausted. The Mr. Reagan who meets Mr. Gorbachev this week is a man who has used up his line of credit at the bank. He can borrow no more.
Both the leaders of the superpowers are in the position of needing a respite from the costs and strains of the cold war.
Both have reason to recognize the limits on their power and range of power. Which is why they are doing some limited and relatively minor arms control business with each other this coming week.