The Byzantine Empire, based mostly in Anatolia and Greece, spent the entire middle ages with more or less the same land, able to expand only a bit in the 10th century, and otherwise being mostly on the defence. Yet when the Ottoman Empire rose and took over the former Byzantine lands, they then proceeded to conquer the rest of the eastern meditteranean and the balkans. What were the differences between the Ottoman and Byzantine Empires, and their larger geopolitical situations, that allowed the Ottomans to expand so hugely when the Byzantines hadn't been able to?