Based on the warring between Shia-dominated Iran and Saudi Arabia as bordering neighbor to Yemen -- which still has a small majority of Sunni -- suggests this to be a logical step in the centuries long conflict between these "nice" folks.
The notion that there is much modernity in any of this -- excepting the weapons with which the age-old conflict is being fought -- is perhaps fallacious.
Huntington's thesis of a "clash of civilizations" (1998 book) comes to mind. Its longer title adds "the Remaking of World Order" and that seems an apt observation now. Sunni versus Shia inside of Islam, and Islam against the Christians and the Jews, as has been true for centuries.
Sunni versus Shia = moslems killing each other since the 7th Century.