Moscow justified the invasion of Czechoslovakia in 1968 with the socalled Brezhnev Doctrine. Now, in an article by Boris Ponomarev of the Party Secretariat, Moscow has updated the Brezhnev Doctrine to justify the invasion of Afghanistan. And a comparison between the two lines provides a gauge as to both Russia's motives and the American response.

The Brezhnev Doctrine took as its focus the world of communist, or as the Marxists call them, socialist countries. It was addressed to the circumstance that Czechoslovakia had produced a communist government that started to edge slowly toward democratic practices and ties with the West. It asserted that in such a case, Russia had a right to intervene. As Brezhnev put it in a speech delivered in warsaw on Nov. 12. 1972:

"When external and internal forces hostile to socialism try to turn the development of a given socialist country in the direction of a restoration of the capitalist system, when a threat arises to the cause of socialism in that country, a threat to the security of the socialist commonwealth as a whole -- this is no longer merely a problem for that country's people, but a common problem, a concern of all socialist countries."

Ponomarev, well known as the leading Secretariat spokesman on foreign affairs, published his article in the current issue of the party's theoretical journal, Komunist. The text was republished by Tass on Jan. 10. It said: j

"Soviet people, of course, are not indifferent to the socio-political orientation by the various trends in the developing world. The devotees of scientific socialism have no intention of denying their spiritual closeness to the progressive forces in Asia, Africa and Latin America. Sympthy with fighters for true freedom is natural for Marxist-Leninists and internationalists. Where such forces exist and are struggling, they have the right to depend on our solidarity and support."

Striking differences characterize the two assertions. Where Brezhnev talks of the common communist world, Ponomarev talks of Asia, Africa and Latin America. Where Brezhnev strikes a defensive pose, Ponomarev looks toward the exploitation of opportunity.

But these differences do not proclaim a change in Moscow. On the contrary, an underlying consistency characterizes Soviet behavior. Virtually the same leaders who ran Russia in 1968 runs Russia today. In each case the Soviet leadership moved -- by stealth and massively -- to augment Russia's power position in the world. Moscow continues to play by Moscow rules.

What has changed is the world itself. In 1968, the communist bloc was unraveling and the Russians had to intervene to arrest the deterioraton. Now the unraveling takes place on the edges of the communist world, and the Russians step forward to support what Ponomarev calls "revolutionary movements . . . aimed at overthrowing . . . pro-Western leadership." In that connection, he cites countries around the Indian Ocean and the Persian Gulf -- notably Iran, Afghanistan and Ethiopia.

The United States has, of course, perceived that the Soviet invasion of Afghanistan constitutes what President Carter has called a "radical new departure." But has the administration perceived that the essence of the new departure is that Russia has hooked itself to revolutionary movements in several countries that are falling apart? Has it understood that the Russians are now treating the course of events in those regions as a matter of vital national interest?

So far, the evidence is ambiguous. The full definition of American policy awaits negotiation with friends in the area and in Europe and Japan. But a comparison of American behavior with Soviet behavior yields a discouraging outlook.

Where the Russians moved on their own, the United States looks to allies.

Where the Russians acted rapidly, the United States delays. Where the Russians engaged forces directly, the United States projects a buildup, and acts on such indirect matters as the olympics and grain shipments.

The balance, in short, seems unequal. The Russians have asserted a vital interest; The United States has drawn back. The Russians are abreast of events that are on the move. But the president has still not positioned American forces in ways that put forward the thin red line and engage, against future Soviet moves, the deterrent.