I'm not sure why you say 'permissible/obligatory' here.
that something is 'allowed' has no relation to that it is 'required' (to use synonyms)
it should be more like
It is permissible that P
It is obligatory(/It ought to be the case) that P
I'm 'permitted' to drink coffee, but I am not 'obligated', -- i can't say 'I'm permitted/obligated to drink coffee'
it is only if I am obligated to drink coffee that I 'ought,' and that I legally or morally 'can' doesn't mean I 'ought.'
I thought my intention in using "permissible/obligatory" was clear enough. "It is obligatory/permissible that P" is used as an abbreviation for two sentences "It is obligatory that P" and "It is permissible that P".
a moral principle may be 'thou shall not kill' but a soldier 'ought' disobey it. the moral principle isn't coextensive with a command to obey it, even if the soldier grants that as a good principle, he doesn't feel obligated to obey it, as I agree an apple a day is a good principle, but again there is no justification to force me to eat an apple every day simply because I accept that it's good for me.
no moral principle is co-extensive with a command, since moral principles can be true or false, but a command cannot properly be said to have a truth value.
more importantly, the soldier example
at most shows that "it ought to be the case that noone kills a human being" is not a true moral principle as stated. if one really thinks that a person (eg. a soldier) is permitted to kill in some (conjuction of) circumstances C, then the correct principle will be "it ought to be the case that noone kills a human being unless C holds". the soldier example therefore does not lead one to think that ought-statements do not oblige people to do certain things.
sure, but if you correct your first statement about rights, then that 'its permissible for you to drink alcohol' doesn't obligate you to any action---it no where says 'you are obligated to use your freedom to drink or not to drink alcohol, to drink alcohol.'
permissive moral principles ("ıt is permissible that P") do not demand any action by an agent. whereas mandatory moral principles ("it is obligatory that P" or "it ought to be that P") demand that the world be a certain way (the way described by the embedded sentence "P"). if the world is already as P says it is, then there need be nothing that an agent actively does in order to obey the moral principle. but if the world is not as P describes it, then an agent will have to actively bring it about that P.
for a time, in America, it sodomy wasn't permissible, but obviously that didn't have anyone obligated to follow it. I don't know where you get the idea of obligation from, even in an 'ought' statement, the 'ought' is the obligation and you haven't explained why someone 'ought.' we 'ought' not kill? why? at least 'its not permissible to kill' is an answer, 'your ought not sodomize, because it is not permissible' is an answer, but there is no moral justification here any more than 'you ought not keep more than 14 slaves' was moral simply if it were a law. the ought comes from nowhere
i have no good answer to the question about the ultimate ground of the truth of certain moral principles. i am inclined to say that they are basic and cannot be explained using morally neutral concepts. but this is a big question and i don't think that we can
only accept that there are some true moral principles unless we have an explanation of the truth of any true moral principle from morally neutral concepts (which is what i believe you were asking for).
i think it is clear enough that there are some true moral principles, but which moral principles are true is another question. the examples concerning sodomy and slavery above do not show that there are no moral truths, but only that what were commonly believed to be moral truths were actually false. seeing what moral principles are the true ones is a very difficult feat, but it is irrelevant to our purposes here. we are only concerned with the question about the existence of true moral principles and with whether or not we are obligated to follow what is said by true ought-statements.
a moral principle says 'you can defend yourself when someone tries to rape you' but does that mean you were immoral---you didn't do what you 'ought'---if you were paralysed with fear or chose not to resist not wanting to make them more violent when that happened? I don't think a good suggestion or an inclination is an obligation
Suppose "it is permissible that P" is true. Now, if an agent does not bring about that P, should we think that he has done something wrong? No. Permissibility statements tell one what one
can do, and not what one ought to do. In the case at hand, if the person does not defend herself, she has not failed an obligation even if she is not paralyzed etc. I have not said anything that I believe implied the contrary.