how does a nation invade another nation or region and start war?
For War, check out any number of threads (mostly Stickies) in either Nationstates or International Incident. It's all joyful and useful, but basically you roleplay the war.
For Region Invasion, that a Bad Thing. Don't do it. Basically you swamp a region, announce one of you as UN Delegate and there you go. But as I said, its a Bad Thing.