For the Sudetenland it was pretty popular for most Germans, basically after WWI German and Austria (Germanic peoples) lost huge tracks of land to Czechoslovakia (slavic people) and since WWI started due to hostile relationship between the two groups (in the most basic of terms) most Germanic people in the Sudetenland didn't want to be governed by Slavs and Germans in German weren't too keen on the idea ether. I do apologize but I never took the time to learn if any kind of referendum was held for legality sake so I'd like to know that as well.