In which case, any country willing to fund a military and start a war would dominate the world.

Slavery and the death penalty are poor analogies, and not just because slavery was stopped through quite a bit of violence, and the death penalty/imprisonment exists as an alternative to vigilante justice only by virtue of the state's monopoly on violence.

"If everyone just ..." is always silly, but it's especially silly when it comes to the belief that order can be enforced through anything other than -- ultimately -- the threat of violence. Everyone will not "just".