Why is the byzantine history compared to an accordion?

Its territory expanded and retracted several times, like an accordion.

Explanation:

The byzantine empire has a long story of war and gain and loss of its territory. During the reign of Justinian the Great (527-565), the empire achieved its greatest territorial expansion, after conquering most of the old Roman Empire territory, including Italy, Rome and North Africa. Two centuries later, they lost it against the Seljuk Turks, in 1071.

However, the empire was able to recover, and by the 12th century, Constantinople, it's capital, was the wealthiest city in all Europe, and continued to be so, until it was sacked by the crusader armies in the fourth crusade.

It expanded and collapsed like an accordion