You dont need outside support to declare independence, it is of no concern to those in Catalan what the US or anyone else thinks. The best course of action for the Catalan parliament would have been to call an election and then if they won it with a decent majority used that as their mandate to declare independence. As it is they dont have a solid mandate so it is easier for Spain and the EU to ignore them, however all those Catalans who didnt support independence may well change their minds if the spanish army comes rolling into Barcelona.