And to reverse the phrasing but keep the premise and conclusion intact, one could say "If used, nuclear weapons would likely cause great harm to civilian populations. Therefore, countries are wrong to own nuclear weapons."

Let me know if I have this right. "It is wrong for countries to own nuclear weapons. After all, if used, it is likely these weapons would cause great harm to civilian populations.

Premise: If used, it is likely [nuclear weapons] would cause great harm to civilian populations.

Conclusion: It is wrong for countries to own nuclear weapons.