While it is too early to say that the Balkans are reverting to historical type as the tinderbox of Europe, 30 years of rigidly enforced stability under new aging Communist leaders may be coming to an end.

The brittle nationalist passions and big-power rivalries that caused the Balkans to trigger World I are stirring again, with the four Communist states increasingly at odds with each other or with the Soviet Union.

At the same time, the Balkan Communists are all busy wooing their southern neighbor, capitalist Greece.

The attention paid to improving relations with Greece, which is a member of NATO, is a reflection of the many strained nerves exposed in the Balkans over the last year. Romania has differed repeatedly with the Soviet Union on foreign policy issues; old enmities between Bulgaria and Yugoslavia have flared up, and Albania has irrevocably ended its 17-year alliance with China.

In the last few weeks Yugoslavia and Romania have been weept by repeated rumors of imminent Soviet troop maneuvers in Bulgaria -- an event that would be construed as a form of pressure against them. The reports have been angrily denied by the Bulgarians but they do illustrate how easily tension can rise in southeastern Europe's once turbulent Balkan peninsula.

With the accent among them all on friendship with Greece, this weekend Greek Prime Minister Contantine Karamanlis is in Yugoslavia for talks with President Tito on Balkan cooperation. On Sunday he flies to Romania to meet President Nicolae Ceausescu and next month he is due to receive Bulgaria's Communist Party chief Todor Zhivkov in Athens.

Even Albanian Enver Hoxha, the world's last surviving Stalinist ruler, has been attempting to strengthen trade and diplomatic ties with Greece -- despite a long-running territorial dispute between the two countries.

Foreign analysts here attribute some of the present stirrings in the Balkans to the visit to Romania and Yugoslavia last August of the Chinese leader, Chairman Hua Guofeng (Hua-Kuo-feng). The tour drew a storm of criticism from Moscow, which accused the Chinese of meddling in its own sphere of influence. The visit also precipitated China's rift with Albania -- which operates on the principle that Yugoslavia's friend is its enemy -- and encouraged Ceausescu to step up his defiance of the Soviet Union.

A senior Western diplomat commented: "It would be argued that all this would have happened anyway.But I think Hua's presence here brought the latent tensions to the surface and demonstrated the overriding importance of nationalism in the Balkans despite over 30 years of Communist rule"

By this interpretation, events over the last year have illustrated the central theme of Balkan history: How to reconcile the national aspirations of fiercely independent ethnic groups with the often cynical foreign policy interests of large supra-national empires. Each current Balkan ruler has struck some sort of compromise between the conflicting pulls of nationalism and obedience to the region's dominant power, the Soviet Union.

Bulgaria's national interests have been identified with unswerving loyalty to Moscow. The love for Mother Russia of Bulgaria's 9 million Slavic inhbitants is officially described as "eternal and unshakeable." Just how long this state of eternal love will last is open to question, but for now it serves both partners well.

More than other nations of Eastern Europe. Bulgaria undoubtedly has benefited from massive Soviet technical and economic assistance. Standards of living have risen stedily. The alliance has also meant that Bulgarian nationalism can safely be directed against the country's traditional enemy Serbia, now the largest of Yugoslavia's six republics.

The Soviets have been able to hold up Bulgaria's subservient gratitude as an example of the kind of relations that should exist among Communist countries. With the possibility of unrest in neighboring Turkey, Bulgaria also has assumed considerable strategic importance. Last November the world's most modern ferry line was inaugurated between Soviet and Bulgarian ports.

Romania, with a population 21 million, likes to describe itself as "an island of Latins surrounded by a sea of Slavs." Under Ceausescu, Romania has continued to seek a greater degree of independence from the Kremlin while realizing that in political and military terms it has no future outside the Soviet bloc.

The relative milaness of the Soviet reaction to Ceausescu's repeated gestures of defiance, notably his public refusal to increase defense spending in line with the rest of the Warsaw Pact, has surprised some observers. One explanation lies in Romania's 830-mile common border with the Soviet Union. It is virtually detenseless. The Romanian Army, equipped with obsolescent Soviet tanks and fighter aircraft, would be able to offer little resistance in the event of a Soviet-led invasion.

Another reason for the Kremlin's forebearance is that, unlike Czechoslovakia in 1968, it could not be claimed that socialism is threatened in Romania. Ceausescu combines his diplomatic saber-rattling abroad with harsh doses of repression at home.

The Kremlin's ability to reassert its authority in Romania any time it chooses is reflected in a joke originally told by a Soviet journalist who asked rhetorically how long it would take the Red Army to reach Bucharest should an invasion become necessary. His reply: "If the Romanians resist, it will take our boys 24 hours to occupy Bucharest. If they don't then it will take them a week because of all the welcoming receptions they will be obliged to attend."

A Soviet invading force would face much tougher problems in Yugoslavia, with its rugged mountain terrain and highly efficient armed forces. But Yugoslavia, with a population of 22 million, is itself a fragile multinational state. It is made up of many different ethnic minorities with a long history of mutual hatred and bloodshed -- a situation the Soviet Union might exploit after Marshal Tito's death.

Tito's genius as a politician has been to play internal pressures against external ones, exploiting his prestige as the sole surviving founder of the nonaligned movement. By standing up to Stalin in 1948, he won tremendous popularity within Yugoslavia which helped to create the image of a united country. The possibility of Soviet interference is still a strong incentive to Yugoslav unity.

In the hands of less skillful successors, the same policies could backfire. Yugoslav officials are therefore quite sincere when they insist that it is in their country's vital interests to maintain good relations with the Soviet Union. This explains why, despite suspicions in Belgrade that the Kremlin is trying to subvert the nonaligned movement, Tito is bound to make some move sooner or later to patch up the present quarrel with Moscow.

In fact the only Balkan Communist country that appears to be going entirely its own way, oblivious to Soviet pressure, is Albania, strategically wedged between Yugolavia and Greece at the mouth of the Adriatic.

Its ruler, Hoxha, now depicts his country as the remaining bastion of world communism. He launches regular blasts against the heretics in Moscow, Peking, and Belgrade -- all of them, in his view, in ledgue with Washington.

The price for such maverick independence however, has been the almost total isolation of Albania's 2.5 million citizens. Hoxha apparently feels that his citadel would rapidly crumble if exposed to foreign influence. During its turbulent history, Albania has alternated between periods of fierce independence and dependence on a foreign power.

Among them, the four Balkan Communist leaders have been in power for 109 years -- an average of over 27 years each. Ceausescu, 61, the youngest, became Romanian leader in 1965. Zhivkov, 67, was elected first secretary of the Bulgarian Communist party in 1954, while Tito, 86, and Hoxha, 70, came to power as a result of leading guerrilla uprisings in World War II.

They have survlved by blending compliance with defiance, creating at least outwardly cohesive societies in a notoriously unstable region.

What has not been solved, however, in any of the countries is the problem of the succession. The preeminence of the four leaders issuch that the death or removal of any one of them could create a dangerous power vacuum difficult to fill. They have all refused to designate a successor. Indeed, each has taken care to eliminate most of the best qualified candidates.

Ironically, the very success of the Balkans' Communist leaders at remaining in power could pose the biggest single threat for the region's future stability.