I guess I don't really understand "national security is compelling". National security, like basically everything else, is a matter of degree. There are presumably compelling and uncompelling national security risks. If the court is weighing some amount of speech restriction against some amount of national security risk, presumably the size of the risk matters?

What are the governments burdens here? Can they merely assert there's a security issue here or do they have to prove it? Does it have to pass some kind of severity test in order to restrict speech? I'm genuinely curious what the case law says on this stuff.