When a football team picks up a couple of first downs, it acquires what the announcers call "momentum" and becomes truly dangerous. So it is with the Soviet Union in foreign policy.

By luck or design, the Russians have recently been scoring some big gains. Stopping them before they acquire momentum has suddenly become a grave problem for the Carter administration.

The big new push forward came in Afghanistan. A military coup last week unseated a nationalist regime. The new government is headed by a Communist prime minister, and of the 13 members of the Cabinet, 11 are Communist.

Because Afghanistan is a remote country, bordering on Russia, there is a disposition to write off any changes as largely local. But it is ominous that Washington had no advance knowledge of the change, and that Russia recognized the new regime before its composition was announced. Even more so that the new prime minister, Nur Mohammed Taraki, is known for his espousal of Afghan claims on the tribal population of eastern Iran and western Pakistan - what is known as Baluchistan. For a pro-communist regime is in good position to make trouble for both Pakistan and Iran in that area, and the area, as the shah keeps pointing out, virtually gives access to the Persian Gulf and Saudi Arabia.

Suspicions of Soviet motives in Afghanistan find an equivalent situation in Southeast Asia. The Vietnamese, who are linked with Moscow, seem to be working up a struggle with the Cambodians, who have ties with Peking. Events there are obscure, and local feuds go back to the beginning of time. but it is certainly not fanciful to see the Vietnamese, who would surely win a war, extending Moscow's banner to a sector of Southeast Asia where China used to be dominant.

In the Horn of Africa, the Russians have recently scored two big gains and set and stage for a third. They and their Cuban allies have helped the military regime in Ethiopia score a smashing triumph over an invasion of the Ogaden province by forces from Somalia. Apart from being high in Addis Ababa, they have made the regime in Somalia highly vulnerable. Moreover, the Russians and Cubans can probably increase their influence in Ethiopia by helping the regime - either diplomatically or militarily - to subdue the secessionist movement in Eritrea with its ports on the Red Sea. Besides scaring the Saudis even more, a strong Soviet position in Ethiopia provides an indent for further gains in Africa.

More tempting still for the communists is Western Europe. The victory of President Valery Giscard d'Estaing and the center in the French legislative elections last month looked like a terrible setback for the Communists throughout the area. But I for one do not see how the chaos in Italy arising from the kidnapping of former premier Aldo Moro can be halted without the Communists' creeping closer to - and maybe into - office.

I am well aware that openings for Soviet penetration exist at almost all times. Equally that the history of the postwar era is largely a history of missed opportunities for the Russians.

But I don't think anyone can fairly accuse me of being alarmist. Moreover, in the past there was organized opposition from this country and its allies. Now, the United States, still suffering from the Vietnam complex, is clearly divided with respect to opposing communist advances. Certainly there is no disposition to send American boys anywhere.

Perhaps the Europeans and Japanese have more stomach for standing up to the Russians. But not without some U.S. leadership - as Chancellor Helmut Schmidt made plain in Hamburg last week, and as Prime Minister Takeo Fukuda has been saying in Washington this week.

What is perhaps most discouraging is that the West now has as small a carrot as it has a stick. Some people may think that the Russians will draw back rather than risk the chance of not getting a new armscontrol agreement. But it must be clear to Moscow, as it is to every close observer in Washington, that there is an increasingly slim chance of getting an accord through the Congress before the election this fall - and no great certainty even after that. That Russians, in other words, have almost as little to gain by behaving well as they do to lose by asserting themselves.

So the president has to start doing something. Even those of use who have been most keen for detente must now see that, unless given some strong warning, the Russians will talk themselves into the amost dangerous of all positions: the self-intoxicating position of believing that they can get away with anything.