of the Danube, though they were always overrunning into Upper Moesia, the
modern Serbia. The Slavs, whose numbers were without doubt very large,
gradually settled all over the country south of the Danube, the rural
parts of which, as a result of incessant invasion and retreat, had become
waste and empty. During the second half of the sixth century all the
military energies of Constantinople were diverted to Persia, so that the
invaders of the Balkan peninsula had the field very much to themselves. It
was during this time that the power of the Avars reached its height. They
were masters of all the country up to the walls of Adrianople and
Salonika, though they did not settle there. The peninsula seems to have
been colonized by Slavs, who penetrated right down into Greece; but the
Avars were throughout this time, both in politics and in war, the
directing and dominating force. During another Persian war, which broke
out in 622 and entailed the prolonged absence of the emperor from
Constantinople, the Avars, not satisfied with the tribute extorted from
the Greeks, made an alliance against them with the Persians, and in 626
collected a large army of Slavs and Asiatics and attacked Constantinople
both by land and sea from the European side, while the Persians threatened
it from Asia. But the walls of the city and the ships of the Greeks proved
invincible, and, quarrels breaking out between the Slavs and the Avars,
both had to save themselves in ignominious and precipitate retreat.

After this nothing more was heard of the Avars in the Balkan peninsula,
though their power was only finally crushed by Charlemagne in 799. In
Russia their downfall became proverbial, being crystallized in the saying,
'they perished like Avars'. The Slavs, on the other hand, remained.
Throughout these stormy times their penetration of the Balkan peninsula
had been peacefully if unostentatiously proceeding; by the middle of the
seventh century it was complete. The main streams of Slavonic immigration
moved southwards and westwards. The first covered the whole of the country
between the Danube and the Balkan range, overflowed into Macedonia, and
filtered down into Greece. Southern Thrace in the east and Albania in the
west were comparatively little affected, and in these districts the
indigenous population maintained itself. The coasts of the Aegean and the
great cities on or near them were too strongly held by the Greeks to be
affected, and those Sla