Why did some nations declare war on other nations?

A declaration of war is made usually by the national government and professionally declares the war on another nation. There can be many reasons for this. Popular reasons for declaring war are for religious purposes, gain of territory, defense, or even sometimes for gain in economics. In the United States Constitution it states that Congress has the power to raise armies and declare war.

I'd say: Maybe for space problems, or if a nation stole from another nation. Or desperation.