The swiss cantons only hold limited sovereignty. (their Constitution say this clearly) and as mentioned any change is the number of cantons in the federation, do require all cantons to be included in the decision. But it is an interesting country, with its army being one of the only real organizations that exist at the federal level. Naturally, in reality sovereignty is not a yes/no question. In many ways the EU have a lot more influence on how laws are here in Denmark, then the federal government have over state laws in the US. And there are a lot of things Denmark can't do. But it is primarily a lot of minimum rules in regulating the market. and allowing the free movement of goods and workers across borders within the EU. In the typical understanding of sovereignty the ability to conduct you own foreign politics and send ambassadors to other countries are central. And the EU members can still do that. (GB, Denmark and some others joined the US in the wars in both Iraq and Afghanistan... this was not true NATO and not true the EU, but the countries deciding on their own if joining the coalition was the right thing to do) GB is not seceding unilaterally as the south tried. GB is using a specific § in the EU treaty to tell the rest that they want out. This is then followed by a 2 year period where the two parties negotiate the future relationship. Making what they are doing legal.