You have it in reverse. A federal EU enables EU countries to pool sovereignty and strength to protect itself from the US and China. It neither needs to or wants to challenge anyone. A federal system allows EU wide decisions to be made quickly and without petty parochial concerns.
Constituent states rightly can choose to make their own decisions in matters like this, it hardly effects anyone else, and they can choose to collaborate.
Constituent states rightly can choose to make their own decisions in matters like this, it hardly effects anyone else, and they can choose to collaborate.