Ummm, you are not wrong, but you are also not correct.
I am finding it impossible to untangle what you wrote into truths and fallacies.
You have complete misunderstood what is being said in the Bhagavad Gita.
However, your conclusion that all is one, and thus the battle is always internal, is accurate.
Still, you may find yourself in external conflicts with your fellow beings.
And then you must realize that the battle is the same within as without,
and you do not fight your foe with anger in your heart, or scorn in your eyes.