I'm confuse about the sentence too because it came from a novel. The context is when a captain is ordering his team to protect the leader.I'm afraid I don't understand your question, hothead. I have the feeling you've parsed the sentence wrongly: "are to" here has a meaning similar to "must".
In (2), do you really think it's "are" itself that provides the modal sense? I wonder why not think of "to", not "are", as the one that provides the modal sense....
In (2) 'are' is a modal verb, in the semantic sense that it changes the modality from fact to duty: we don't know they are protecting, only that they should.
I think "ought to" as well.The key difference between modal 'be to' and modal 'have to' is that 'have to' is usually phonetically fused: you get /f/, /s/, and /t/ in 'have to', 'has to', and 'had to'. So this has been reanalysed into some kind of single-word verb with a new declension (as have 'used to' and 'supposed to').
I want all I have to belong to youI can't think right now of "have + a "to infinitive" being not the modal verb.
But there is a grammatical reason, isn't it...In any case, there's still no phonetic reason to treat modal 'are to' as in any way fused.