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Weakened by civil war and under constant pressure from the crusaders from the west, the Arabs from the south, and the Mongols from the east, it was amazing that the Seljuk Sultanate lasted as long as it did. When it eventually weakened, the small remaining principalities all vied for supremacy. When peace eventually came through the retreat of both the Mongols and the crusaders, one of these principalities rose to dominance and succeeded in building a powerful and extensive empire for the next several hundred years: that of the Ottomans.
In 1301 the leader of one of these principalities and the founder of the Ottoman Empire, Osman, defeated a Byzantine army a few miles from Constantinople. This gave him great prestige and led to the consolidation of Ottoman authority over a substantial area in north-west Anatolia (Turkey). The Ottomans expanded rapidly, absorbing weaker tribes to the east and reducing the weakened Byzantine Empire to just the city of Constantinople by 1351. The Byzantine Emperor attempted to persuade the Pope in Rome that, despite their differences, they had a common enemy, even travelling to Rome in person in 1369 to submit publicly to the Pope in the hope of receiving aid, but with no success.
In 1389 the Ottomans, under Murad I, wiped out a huge combined army of Serbs, Albanians and Poles at the Battle of Kosovo Polje, in present-day Serbia, in yet another defining moment for the West. Shortly after the battle, the whole of Macedonia was incorporated into the Ottoman state. Murad himself was killed in the battle but his son, Bayezid, who succeeded him, ended up laying siege to Constantinople in 1394. It seemed that nothing could stop the Ottoman advance and that the long-awaited collapse of the Byzantine Empire was finally at hand. However, at the last hour it was the Ottoman Turks themselves who were attacked from the east. The capture of Constantinople would have to wait.