One important question that I'm unclear on is how long it takes to fix one of these cables. If it takes months then that is quite a wide window in which an attacker could incrementally take down cables.
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(And if they decide they don't care about plausible deniability, they could use sub-deployed timed mines to take out every cable at once.)
In an actual war, you hit the repair equipment and personnel [1].
(As to the Geneva Conventions note, we're discussing a hypothetical war with Russia. The status quo, including rules of war, are going to be rewritten by the victors.)