WW1 was just plain nasty and I blame that war and WW2 for all the mess the 20th century had to endure the end of the Ottoman, Austro-Hungarian and Russian Empire all fell leading to tiny ethnic based states once all these ethnic and religious groups who had no tension all teared each other apart in a orgy of violence that even 100 years later we still or some us recall those nasty pics the use of chemical war fare
French revolution and the age of nationalism screwed things up. For the Ottomans and Austrians it was the beginning of the end.
When it comes to the criticism of multiculturalism people use the Ottomans and the Austro-Hungarians to point out its failure.
German Empire collapsed due to defeat in ww1. Russian Empire collapsed not due to multiculturalism but due to the Russian Revolution even before ww1 the Russian Empire had so many problems that the empire was a bustling kettle ready to blow out this also includes ethnic groups like Poles, Kazakhs, Latvians and Finns trying to break away from the Empire.
Its crazy how today we are still living with the aftermath and legacy of Ww1 especially in the Middle East.