Europe has been a vassal since 1945. How do you vassalise your vassal?
EU suggested forming an EU army as early as the 1950s! USA was against the EU forming its own integrated military. So was, not a member yet, Britain, historically always scared of a too united continental neighbour. Instead, the Anglo-Saxons basically forced the young EU-predecessors to rely on NATO instead of forming their own big military defence force. How the tables turn.