Witch external proved to be the greatest continuing threat to the byzantine empire in the 15 th century?

The Late Middle Ages or Late Medieval Period was the period of European history lasting from 1250 to 1500 AD. The Late Middle Ages followed the High Middle Ages and preceded the onset of the early modern era (and, in much of Europe, the Renaissance).Around 1300, centuries of prosperity and growth in Europe came to a halt. A series of famines and plagues, including the Great Famine of 1315–1317 and the Black Death, reduced the population to around half of what it was before the calamities. Along with depopulation came social unrest and endemic warfare. France and England experienced serious peasant uprisings, such as the Jacquerie and the Peasants' Revolt, as well as over a century of intermittent conflict, the Hundred Years' War. To add to the many problems of the period, the unity of the Catholic Church was temporarily shattered by the Western Schism. Collectively, those events are sometimes called the Crisis of the Late Middle Ages

the steam engine.

the locomotive.

the telephone.

the telegraph.

the internal combustion engine.

the rifle.

ironclad ships.

electricity/light bulb.