Cuban military advisers have begun arriving in Ethiopia to help the besieged Marxist military government there survive. The country's fragile national unity is being seriously threatened by secessionist movements in two sections.

Western diplomatic sources here said that first contingent of about 50 Cuban military experts arrived in Ethiopia two weeks ago and estimated that about 400 to 500 would be sent there in the next few weeks.

The beginning of Cuban military assistance to the Ethiopian military government appears to indicate that Fidel Castro is prepared to involve his country in yet another major internal African conflict in direct conjunction with Soviet aims and designs on the continent. This time the scene is in northesastern Africa, along the vital Red Sea waterway.

The Cuban action - which follows the arrival of large quantities of Soviet arms in Ethopia, including 150 tanks and armored personnel carriers - seems to be part of a closely coordinated Soviet-Cuban plan to compensate the Marxist regime of Col. Mengistu Haile Mariam for its recent loss of American military assistance!

Last month Ethiopia cut all military ties with the United States, abruptly ending two decades of almost total dependence on American arms and military aid.

The Cuban leader's decision to become involved in the Ethiopian political quagmire was almost certainly made during his visit to Addis Ababa in mid-march. On that trip he held long secret talks with Mengistu and was briefed in detail on the multiple troubles facing Menigstu's military government.

The decision marks Cuba's second military engagement in an African civil war of uncertain outcome and duration. The first one, in Angola two years ago, required the commitment of more than 20,000 Cuban combat troops and ended in the victory of the Soviet and Cuban-backed Popular Movement for the Liberation of Angola over its two pro-Western and South African-backed rivals.

But Cuba has been obliged to keep thousands of troops in Angola to assure the survival of the Popular Move- ment government, which is still facing a threat from dissident guerrillas in the north and the southern third of the country.

Castro's commitment to the Ethiopian military government could eventually prove even more demanding and costly than that in Angola. This time the Cuban leader is pitting his prestige and his country's limited military manpower against secessionist and opposition groups in Ethiopia that have strong backing from the oil-rich conservative Arab states.

Thus, Cuba, is becoming, for the first time, directly involved in the mainstream of Arab as well as African politics.

There is considerable debate in Arab and Western diplomatic circles here in Addis Ababa, the Ethiopien capital, about just how far Castro intend to go in his bid to bolster the Ethiopian Marxist government.

Arab diplomatic sources here believe that Cuba is prepared to send combat troops as well as advisers to Ethiopia and that several thousand Cuban soldiers are on their way there. But western diplomats in the region region doubt that Catro has made up his mind to send more than military advisers, at least at this early stage.

It is thought here that the first task of the Cuban military advisers will be to help command and coordinate operations of the huge volunteer army the Ethiopian military government is now forming. The army, which the government says eventually may total 10,000 "peasant warriors," is expected to fighting the northern province Eritrea and the eastern Ogaden region, where separatis groups have taken over most of the territory and even may of the smaller towns, and against the seized control of most of Begemdir Province in northwestern Ethiopia.

The "peasant army" has been organized to bolster the regular, standing one, which now numbers more than 50,000 troops. The regular army's morale and discipline have been shattered by the revolution that has gone on in Ethiopia since the military deposed the late Emperor Haile Selassie in September 1974, and by the long unresolved war in Eritrea.

Between the fighting in the Agaden and Eritrea, the Ethiopian army is estimated to have lost 11 battalions around 5,000 soldiers, in the past year alone. Six of the battalions were disable in the Ogaden, according to Somali and Western diplomatic sources and the other five in Eritrea, accord.

The Cubans are likely to find that the fighting in Ethiopia is far differing to French military estimates.

[TEXT OMITTED FROM SOURCES] ent from what they experienced in Angola. Unlike the Angolans, the Ethiopians have traditionally produced tough soldiers, and the army has a long military tradition that in cludes a distinquished record of service with U.N. forces in the Korean War.

The guerrillas fighting for the independence of Eritrea operate in extremely rugged mountainous terrant and have proven to be even tougher fighters, despite the Etihopian armys superiority in arms and aircraft. The Eritrean rebels have been conducting their guerrilla war for almost 16 years, are well dug-in Vietcong-style throughout the province, and actually outnumber the army's forces there.

The Somali insurgents in the Ogaden region, which covers perhaps a third of Ethiopia, have also quickly proven a match for the thinlydeployed Ethiopian army and are extremely well armed.