Can you explain what's the reason? You can say positive as well as negative reasons.
I think the most common reason for war is the urge to capture resources owned by another jurisdiction. For example, the USA frequently goes to war to secure access to energy resources.
Reason? I don't think there is enough reason to support war! The truth is the only ground why nation wants to fight the other one is because of greediness manner for no good government would love to kill the people in other nation just to conquer that place for personal gain. The good country would love peace talk rather than using harmful thing to get what they wants.