And they had the right to establish that right by the laws and usages of war.
That sounds like circular reasoning. It sounds like you are saying they had the right to leave *IF* they had the power to win a war. In simple terms, "might makes right."
That is not how I understand natural rights. Your argument applies perfectly well to slavery. A slave cannot be free if he cannot overcome a superior force holding him in bondage.
“ Your argument applies perfectly well to slavery. A slave cannot be free if he cannot overcome a superior force holding him in bondage”
And so.
How were the slaves eventually freed?