Today, maps mattered more than religions.
The Ottoman Empire was one of the greatest empires in history, arising in the late 13th century as a mostly Islamic entity, ruled by a succession of sultans (also called caliphs), and based in Constantinople (modern-day Istanbul). At its zenith, the Empire controlled most of the land facing the Mediterranean Sea (except Italy and Tunisia) and the land surrounding the Black Sea, as well as much of the Arabian Peninsula. It hastened its own demise in 1914 by siding with the Germans in World War I, which yielded a few military victories, like the Battle of Gallipoli, but lots more defeats. It gave the world the massacre of a half-million or more ethnic Armenians, because Armenian units had helped the opposing Russian Caucasus Army.
The writing was on the wall by 1917, when on this day British Foreign Secretary Arthur James Balfour wrote to Baron Rothschild that the British government supported establishment of a Jewish homeland in Palestine. The Jewish nationalist movement, called Zionism, had been lobbying for just such support since the late 1800s. Once Germany surrendered and signed the Treaty of Versailles in 1919, the Ottoman Empire followed suit and signed the Treaty of Sevres in 1920. Little did it know that Britain and its allies had secretly agreed to disassemble the Empire, and run some of the most financially lucrative parts themselves. In all, 39 countries were created, and while Israel was on the list submitted in the Treaty, the area that would become it and Jordan would be ruled under a British Mandate until Israel declared its independence in 1948. Most of the countries thereby created have spent their time since fighting one another, or themselves.
Drawing lines to create communities is the easy part.