JUNE 28 has been a fateful date in south Slav history. Six hundred years ago, the Battle of Kosovo brought medieval Serbia under Ottoman rule. That lasted until 1912.
In 1914 in Sarajevo, a Serbian student assassinated the heir to the Austro-Hungarian imperial throne and precipitated World War I.
Thirty-four years later, Joseph Stalin expelled the Yugoslavs from his Cominform, the international organization through which he ran Eastern Europe.
Though all three incidents are distant memories, the first and third remain vivid in Yugoslav minds.
The year 1389 has always been a sensitive date for Serbians, but never more so than in the past two years. A new and extremely nationalist Serbian Communist Party leadership has exploited the Battle of Kosovo in a bid to make their republic again the dominant power, albeit in modern-day Yugoslavia's federation of ``equals.''
On Wednesday, at least 300,000 Serbians flocked to the nearby Kosovo Polje (field) where their forebears perished under Turkish swords. This time they were also celebrating recent constitutional changes restoring Serbian hegemony over the province of Kosovo, where the ethnic Albanian population outnumbers local Serbians by eight to one.
Croatia and Slovenia, smaller in population but economically stronger than Serbia, view this development with open dismay.
And there are many Yugoslavs - even among the Serbians here - who feel that the country has more reason to appreciate a latter-day victory rather than make so much of a medieval defeat.
That victory was on June 28, 1948, when Stalin excommunicated the party of Yugoslav leader Josip Broz Tito and so - unwittingly - made Yugoslavia independent. For Yugoslavs at large, that was indeed a ``day of liberation.''
Stalin was apparently unaware of what he was doing. As his Cominform edict said, he expected the ``healthy [i.e. pro-Moscow] elements'' in the Yugoslav party to throw Tito out. The Russian ambassador in Belgrade told Stalin this would happen.
A Pravda correspondent in Belgrade that hot summer said he was confident that Tito was ``like an overripe apple. You shake the tree and it falls.''
Stalin moved tanks menacingly on Hungary's and Romania's borders with Yugoslavia. But the ``healthy elements'' did not emerge. The few there were, Tito adroitly put out of harm's way on an Adriatic island. The ``apple'' did not fall.
Those were dangerous days for Tito. If Stalin had intervened militarily, Tito's position would have been perilous in the extreme. The West was not merely skeptical of the ``break'' with Moscow. Tito's own aggressive behavior over Trieste in 1945 had scarcely endeared him to the Western Allies.
Military opinion was that, if Stalin struck, Tito would certainly have resisted, but that at best his war-weary partisans could delay the Russians for a month before they took Belgrade. But Stalin did not strike, and Western aid bolstered the new independence.
Of Tito's nerve, there can be no doubt. He had openly defied Stalin in a way no communist leader had done before and lived. A seed, in fact, was sown among communists which periodically germinated in Eastern Europe from 1956 to this day when Hungary and, to a lesser extent, Poland are committed to a reform course.
Stalin accused Tito of downgrading the Communist Party. A statement this week from Hungary's newly appointed chairman, the veteran social democrat Rezso Nyers, is, in this context, a truly historic declaration:
``We are doing away with Stalinism and the dictatorship of the proletariat and adopting parliamentary democracy based on absolutely free elections.''
There is little more ``untouchable'' in communist mythology than the ``dictatorship of the proletariat.''
The Nyers remark is sure to draw more fire from the East bloc's anti-reformers who, like East Germany, are already sniping at Hungary in terms identical to those leveled at the Prague Spring reformers in 1968.
Against this background, Yugoslavs would seem better off celebrating - and improving - long-established reforms rather than calling them into question by reviving national rivalries and ``superiorities'' of the past.