Answer:
Since they were essentially Romans, they had inherited the
military know-how and discipline from the Roman Empire. The wealth
of the Byzantine (Eastern Roman) Empire also helped as the Empire
was able to afford a massive army and navy, provide excellent
training and pay the soldiers good wages and regularly.
Just like the old Romans, the Byzantines co-opted the defeated
tribes into their ranks thus creating a multi-ethnic force that saw
it in their interest to fight for the Empire because the Empire
provided safety, trading opportunities and prosperity.
Capable generals (Belizarius, for example) and emperors
(Justinian), just like in the old days (think Marius, Sulla,
Pompey, Caesar, Agrippa, Marcus Antonius...), played no small part
in strengthening the morale and inspiring obedience. Failure was
punished (see "decimation" in Old Rome) and success rewarded.
As the time went by, the power of the Byzantines waned. From about
the early 1200s (when their capital was sacked, 1204), the strength
of the Byzantine Empire began to decline rapidly until the (final)
fall of Constantinople, in 1453.
Explanation: i googled and copy and paste
