This article deals with War as it relates to NationStates. For more general information, see the Wikipedia article on this subject.
Under the accepted rules of warfare, each combatant states his actions and waits for the other player(s) to respond and post relevant losses. Posting losses for your opponent is considered godmoding, and is considered to be Bad form.
Some nations choose to go to war on an army-by-army basis. Many of them purchase military components from Storefronts, others create their own militaries entirely internally. The size of any nation's military presence is often determined by use of Calculators, and is generally assumed to be a small percentage of that nation's GDP. Pitting force against force is sometimes referred to as number-wanking.
Other wars are fought on a diplomatic level, usually as a series of announcements, communications, or conversations by the principals of those nations and their allies and enemies. These are usually referred to as Character RPs.
If Nation A declares war on Nation B who does not wish to fight, Nation B may ignore the original beligerant by simply not responding to the declaration. Occasionally n00bs will declare war on another nation who does not participate in forum interplay. These topics should be ignored by other players, as they are also considered to be godmodding.