Byzantium/Constantinople was conquered in 1451, but the empire continued to decline starting around the 18th or 19th centuries. It officially died in WWI. As for how long it was a major threat to Europe, the empire's prime was the 16th century, under Suleiman the Magnificent.
<span>an expert in or student of history, especially that of a particular period, geographical region, or social phenomenon.</span>
They did not created reasonable scheduling expectations
Authority gives a state the right to use its power
