This is a really subjective question. Some would argue that the American Revolution produced more good than harm, because it created the U.S. Some would say the American Civil War was worth the death and destruction because it ended slavery. Some would say World War II was worth it because it ended the Holocaust.

It really just depends on who you ask and what you perceive as "good" and what you perceive as "harm."

How are you measuring 'good'? For everybody, for the original aggressor, or what? Based on what starting point? Compared to no war, or compared to one side not fighting back?