We had the military strength to defend our secession, the South did not. Since the South did not have the military strength they should have taken their secession through the Congress.
Who built them, how much they cost, those are just details. You can’t have a sovereign state with foreign troops stationed on your soil without your permission. That’s called being in a state of war, whether the Northerners want to admit it or not.
So we're at war with Cuba?
“We had the military strength to defend our secession, the South did not.”
Ok, the might makes right argument is sensible, but it’s beside the point if one is arguing about the viability of secession, since it can only be determined after the fact. There’s no way to know ahead of time if you will prevail with force of arms. It also bears no relation to whether the states have an inherent right to secession, since they can have a right to do it even if it’s not feasible for them to exercise the right without fighting a war.