The classic just war theory has a great deal to say about justifications for going to war, and about the proper conduct of war once it’s being fought. But it says almost nothing about the aftermath of war. (For the record, I think this whole theory is defunct. There is probably no such thing as a “just war” in the 21st century. Just for starters, how does one make the required distinction between combatants and civilians?)
The weapons of modern warfare often leave horrific legacies that create death and havoc after a war. Think about the radiation after Hiroshima and Nagasaki, the unexploded land mines around the globe, the depleted uranium in Iraq, and the legacy of Agent Orange in Vietnam.