I'd like to offer a historical example from the Islamic world. The city of Rey (Arabic: الري), near/at present-day Tehran, was one of the most important cities in Persia and in the wider Muslim world. Here's how it lost prominence.
It's difficult to overstate how important a cultural center Rey was. Polymath and philosopher Abu Bakr Al Razi was born there, so was theologian and philosopher Al Fakhr Al Razi. Also Abbasid Caliph Harun Al Rashid, and possibly Muslim theologian Al Ghazzali.
In the 6th/7th century AH (12th/13th CE) Yaqout Al Hamawi was a geographer who traveled widely in the Muslim world and kept a travel journal. He notes that Rey was destroyed twice due to sectarian bloodshed.
The people of the city at the time were overwhelmingly Persian Muslims, some Sunni and some Shia. In a first incident, there was civil strife between the Sunnis and the Shias that ended with the Shias expelled or killed, and much of the city ruined.
You'd think that once Rey became all Persian-Sunni, the civil strife would stop, but it didn't. Sunnis belong to four different schools of jurisprudence. A coalition of Shafi3is and Hanafis now turned on the other two schools, and the city was further ruined.
You know what happened next, right? The Shafi3is turned on the Hanafis.
A few years later, Rey was invaded by the Mongols. The Shafi3is colluded with the Mongols and got them to attack the Hanafis. The Mongols obliged, and then turned on the colluders. The city was destroyed and its people enslaved.
The city never recovered its previous prominence, with Tehran (which is nearby) rising and overtaking it in importance.
Moral of this sad story: Once the idea infects a society that "we can only coexist if we're the same", it's a recipe for endless strife and a harbinger of catastrophe. Some societies don't survive the aftermath.
