@gavinisdie @bjn @cstross @Longspeak @JeremyGraeme NATO requires all members to agree. So NATO will be siderailled into a standards-setting organisation and a EU+ will become the main force for security in Europe.
It's bad for the US because the US's ongoing national security will then depend on the actions of a European-only group. The US would have to ask nicely that a EU+ not risk a Russia nuclear exchange with the US, perhaps threatened because EU+ forces are approaching Moscow. Whereas in NATO the US can insist.