The weakness of the Byzantine Empire after the Fourth Crusade and collapse of the Mongol Empire during the Black Death of 1347 allowed the Ottoman sultans to cross over into Europe in 1352 and begin conquering Greece and the Balkans. By 1361, Murad I had captured Adrianople, and by 1386, Bayezid I had taken Sofia (modern Bulgaria).
Although things were going well in Europe, in West Asia the Ottomans were attacked in 1400 by the Mongols, putting their empire back together under their new leader Tamerlane. A lot of the Ottomans' West Asian land was lost. Bayezid was so upset that he killed himself.
Because they were losing their battles in Asia, the Ottomans focused more and more on Europe. They fought many wars with Venice over control of the Eastern Mediterranean shipping routes. In 1453 the Ottoman sultan Mehmed II and his Janissaries even conquered Constantinople (modern Istanbul)
from the Byzantine emperors, ending the last piece of the Roman Empire.