which we don't know

Which is correct:

[color=red]1-An attack would be advisable if defeat were impossible, which we don’t know for sure.
[color=blue]2-An attack would be advisable if defeat were impossible, which we don’t know.

[color=darkblue]3-An attack would be advisable if defeat were impossible, which we don’t know if it is.
[color=darkred]4-An attack would be advisable if defeat were impossible, which we don’t know that it is.

Gratefully,
Navi.

Hi, your first two sentences are fine, although personally I think #1 is more clear.

#3 and #4 need a different word instead of which, “though” or “although” or “still” would all work.

“Decide to attack, if we are sure that our cause/reason for attack is genuine and we wouldn’t be defeated.”
Is this sentence logical and grammatically correct and natural to hear?

I have a couple problems with your sentence. First, it should really have a subject. Writing it as a command is fine when referring to “you”: “Decide to attack, if you are sure that your …” But with we, it really needs to be “We should decide to attack …”

Also, the “wouldn’t” should be “won’t”. I am not sure of the grammatical rule here, I am just going on how it sounds to me. I think it may have to do with the fact that “is” and “won’t” are parallel, so they have to be in the same tense.