In kidnapping former premier Aldo Moro, Italy's Red Brigades' terrorists yesterday pulled off their greatest coup to date - almost as if an American group such as the Symbionese Liberation Army had been able to seize Walter Mondale or Henry Kissinger.

The clandestine Red Brigades terrorists have been striking at "enemies of the proletariat" over the past five years with increasing frequently. By seizing Moro, the man expected to become Italy's next president, they came closer to the "heart of the state" than ever before.

But apart from creating a sensation, the terrorists did not declare any specific political objectives in the kidnapping.

This is largely in keeping with the style of Red Brigades. As the terrorists of the left, they have gone through a Marxist phase in their opposition to the established order. But, according to U.S. intelligence sources, the Red Brigades seem to have gone beyond Marxism. Their members have no identifiable political philosophy except anarchism.

There is little hard intelligence on the Red Brigades. At first, they were a spinoff from the Italian Communist Party and known under the name of "Metropolitan Left."

The Milan-based Metropolitan Left bolted the party in 1969 on the ground that it was too conservative. In 1970, the insurgents changed its name to Red Brigades and two years later went underground to launch attacks on the traditional political parties and society.

Since then, the Red Brigades' victims have included political and civic leaders as well as a few medium-level figures in the Communist Party.

The Communists in turn have launched a campaign against terrorism, arguing that it is "unacceptable in principle" and that Red Brigade violence was hurting interests of the working class. Privately, party leaders were concerned about a rightwing backlash that could hurt the Communists because of their former association with the anarchists.

In claiming responsibility for various political assassinations and armed attacks, the Red Brigade terrorists do not issue specific demands. They seem to run a war of nerves with society. Shooting Italian establishment figures in the leg has become a Red Brigades' trademark.

U.S. officials said they believe that several groups are involved. They are all exceptionally well organized and have become bigger and more effective over the years. Some estimates put the total number of leftist terrorists between 500 to 1,000.

According to these sources, the Italian police have been unable to infiltrate the group or discover the sources of their weapons.

Although more than 150 suspected Red Brigades' activists have been arrested in the past two years, the group has become bolder. They have claimed responsibility for 44 assassinations, 30 kidnapings and countless armed attacks and acts of sabotage.

One of the group's founders, Renato Curcio, 37, has been held by police since 1975. He and 14 other suspected terrorists were scheduled to go on trial last year. It was postponed after the Red Brigades assassinated the president of Turin law society.