Good post!

Another reason for attacking another country is to simply justify your armed forces that could have been stood up to protect against another country, but that threat has deminished or been resolved. I guess this could fall under population growth, but I think it fits better under internal politics and averting civil unrest.

Another issue would be environmental; famine, diease, drought, and earthquakes flooding. These things may cause a peaceful nation to take drastic action to save their way of life. My thoughts generated by your post. Thank you.