The history of the Balkans is long and complex. The Slavs first entered the region in the sixth century, and by the medieval period, the Slavic group known as Serbs had established a formidable kingdom, including the areas now known as Macedonia and Montenegro. But the Turks defeated them in 1389 and the Ottoman Empire then dominated the region. Many Serbs in Bosnia converted to Islam. Most of Croatia (Croats are also Slavs) fell first under the influence of Hungary and then of Austria-Hungary. Most of Croatia remained Roman Catholic; the Serbs were Orthodox Christians.

In the nineteenth century Serbia began to shake off Turkish rule, culminating in the Congress of Berlin in 1878, under which the Serbs gained their independence. But the same congress gave Bosnia and Herzegovina to Austria-Hungary. Macedonia was freed from Turkish rule in 1913, most of it being awarded to Serbia.

In 1918, after World...