When it comes to takeover of foreign lands outside Eastern Europe, Russia generally acts as though it were led by Woody Allen. In place after place, the Soviets poured in big assets and then goofed so badly they were expelled.

But is that going to be the case here in Ethiopia, where the Russians have acquired vast local power that casts a shadow down the Dark Continent and across the horn of Africa to the oil states? The answer, for a lot of reasons, is that the Russians cannot be counted upon to wither away here.

The general expectation that Soviet penetration will breed Soviet expulsion has been developed out of events in Egypt, the Sudan, Somalia and Syria. In each of those countries the Russians poured in vast amounts of military and technical aid.

In each, national leaders used the help for their own ends. When the help had served its purpose - or when the Russians developed cold feet about the military objectives of the host country - Moscow's men were summarily kicked out.

Here in Ethiopia, the Russians have made a huge commitment indeed. They delivered something like a billion dollars in military and economic aid in the past year alone. But they have not made - or at least they have minimized - the kind of mistakes that cost them so dearly in other countries.

For one things, the Russians themselves have not been obtrusively in evidence here. Cubans, transported by the Soviets, have been the visible presence. They get on much better with the Ethiopians than the Russians, who tend to be stand-offish and full of racial prejudice. One Ethiopian told me the Cubans were "free and easy - like the Americans."

More important still, the Russians have not shrunk from backing ethipia's military objectives. The strategic priority of the Ethiopian strongman, Lt. Col. Mengistu Haile-Marriam, was to defeat the combined guerrilla and regular offensive launched by Somalia last summer against Ogaden Province. Russian officers designed an almost classic military campaign - heavy bombardments, encircling movements by armored forces and an air drop at the rear of the Somalis. That enabled the Ethiopians to rout their enemies in the Ogaden, and further entrenched the regime here.

Col. Mengistu now has a second military objective: reduction of the secessionist insurgency in Eritrea on the Red Sea. The Eritrean liberation movements are dug in deep and well armed. While a conservative wing enjoys Saudi and Egyptian support, a far more formidable group has the backing of leftwing Arab regimes.

The Russians, and even more the Cubans, apparently entertain misgivings about crushing fellow Marxists. Col. Mengistu was in Moscow last month, and to Havana last week, to try to organize support for his Eritrean ambitions. It looks as though an effort will be made to promote a compromise between the regime in Addis Ababa and the left-wing secessionists.

But Col. Mengistu is a hard-liner on Eritrea. He will fight unless he gets a very favorable compromise. No one here doubts that, one way or another, the Russians and Cubans will help Mengistu take over Eritrea. Some fighting, probably some heavy fighting, will surely ensue, and it is likely to bind the Russians even closer to the Ethiopian regime.

Russia's willingness to go all out militarily carries an important political component. Partly for its own reasons and partly because of Russian pressure, the Mengistu regime has already established many elements of a communist state. Plans are now in the works for establishing a Communist Party with a monopoly on all political power.

To be sure, Ethiopia is an ancient country with many indigenous religious and tribal traditions. Its bureaucracy was trained in the West, and remains pro-Western in outlook. But my sense is that it will be a long, long time - at least five and maybe even 10 or more, years - before this country can even begin to move out from under Soviet influence. That is not a happy prospect for the United States, since Ethiopia is an important African state, with a powerful martial tradition and a geographical position that makes it easy to lean on neighboring states - notably Kenya to the south, the Sudan to the north, and Saudi Arabia across the Red Sea.

Still, the unpleasant fact is that the United States has no real assets here. Whatever sympathy Washington enjoyed was dribbled away by the Carter administration in feckless efforts that seemed to support Somalia against Ethiopia. So the best the United States can do is hang in here, avoiding further mistakes that go with supporting the conservative Arab states, and hoping that eventually the Russians will begin to make the kind of errors that in the past were par for their course.