This is the idea of a Multi-Speed Europe. This is nothing new, but voices haven't been that loud (except for these moments when Angela Merkel floated the idea rather vaguely and called it "Europe of two speeds".
This is not limited to military cooperation but also extends to economic and societal integrations. In reality, we already do have these cooperations especially at border regions where enablements of Schengen hits the people directly. Why should these cooperations stop at border regions, for example? The eurozone ist yet another example of a different Europe.
Ich mag ja die tiefstehende Wintersonne nicht. Dann sieht man überall den feinen Staub auf den Elektrogeräten und ist verleitet Staub zu putzen.