Pliny and Ptolemy mention the Serbs, and we know that for one hundred years at least previous to 625 A.D. they were at war with the Empire. The Roman Empire was then slowly disintegrating, and in the Balkans there was no power to protect the Romanized Illyria from the northern invaders who in prehistoric times had driven away the aboriginal inhabitants. It matters little whether the Emperor Heraclius invited the Serbs to settle down in the northwest Byzantine provinces lately devastated by barbarians, on condition that they would defend the Empire against the Tartar Avars, or whether he merely accepted the fact that they had entered these provinces and must stay there. He made an agreement of peace with the Serbs--and this marks the beginning of their known history. He desired a buffer State, as the neighbors of the Serbs so often have desired in later times. The lands the newcomers then occupied are the Serb lands of to-day--Serbia, Montenegro, Bosnia, Herzegovina, Old Serbia, Macedonia, Dalmatia, the Banat, and to an extent Croatia and Western Bulgaria--practically the ideal Pan-Serbia, but in this little sketch, so far as it is possible, by "Serbia" is meant the Kingdom of Serbia, at the north of the Balkan Peninsula. The Kingdom of Serbia is bounded by Bosnia, Old Serbia, Bulgaria, Roumania, the Banat, and Slavonia. The boundary rivers are the Danube, on the north separating it from Hungary and on the northeast from Roumania; the Drina, on the northwest from Bosnia; the Save, on the northwest from Croatia and Slavonia; the Timok, on the northeast from Bulgaria. Various mountain ranges on the west separate it from Bosnia, on the south and southwest from Turkey, and on the south and southeast from Bulgaria. Until the tenth century, except Pliny and Ptolemy, the Emperor Constantine Porphyrogenites is the only historian to speak of the Serbs, and he but briefly; yet their history in those three centuries after their arrival was an epitome of their history in later years in the Balkan Peninsula. The general movement was the same. First, a constant struggle on the one side to establish a union of the jupanias and on the other side a constant resistance to such centralization. A jupania may be roughly defined as a county within whose limits lived clans more or less related to one another. The ruler was a Jupan, and it was not strange that the more powerful Jupans should tend to absorb their weaker neighbors. The successful man took the title of Grand Jupan. Jealousy of the Grand Jupan would lead to assassination, dethronement, and decentralization--and then would come a repetition of the violent and bloody story. Another element of disorder in Serbia was the ancient Slavonic rule that a Jupan might be succeeded, not by his son but by the oldest member of his family. It was hardly to be counted against a strong Jupan that he should try to arrange for his son to succeed him--yet this added to the troubles of the Serbs.