NationStates doesn't have a built in war feature. You can roleplay a war in International Incidents, but you can only declare war on people with their permission and it requires a lot of give-and-take — you can't just write, "I HAVE A BAZILLION NUKES! I WIN!" I'd recommend contacting one of the II Mentors.
You can make war in the regional message board in many regions. We used to do that in my region back in the good old days. Before the dark times... before college.