
https://blogs.lse.ac.uk/europpblog/2023/06/19/is-differentiated-cooperation-the-way-forward-for-eu-foreign-policy/

https://ecfr.eu/wp-content/uploads/Ambiguous-alliance-Neutrality-opt-outs-and-European-defence.pdf

https://www.ips-journal.eu/topics/democracy-and-society/the-berlin-blob-7462/