No, that's not why the whole thing started. That is just what Putin told you. And apparently you just assume he is telling the truth, because after all, why would he possibly lie? Has the ring of a similar answer from Trump on the same topic, doesn't it?

Does it really make sense to you, this argument that it was a concern about nuclear missiles that caused Putin to invade Ukraine? Give what I assume is a normal understanding of military armament possessed by the US and by Russia, does this really make sense to you?