...until it was besieged and sacked by Crusaders form Western Europe.
Yup, that was awful.
It rolled back the impressive recovery which the empire had made in the 12th century under the Comnenus emperors, after the disaster at Manzikert in 1071.
Despite the 1204 sack of Constantinople, though, the empire made a strong comeback and held onto a big chunk of western Anatolia through most of the 13th century. Then came the Ottoman Turks.
BTW John Julius Norwich died this month. He was a great Byzantine historian, and a viscount to boot. (The title to which I aspire.)
The Crusaders were going after mohammedans, but didn’t do a good enough job, sadly.