Just to tack a few things on to other people's points, I think at least one reason the amount of nation-state warfare has decreased is because of the effects of

Just to tack a few things on to other people's points, I think at least one reason the amount of nation-state warfare has decreased is because of the effects of colonialization and decolonailization. That is, we're left with a lot of countries with political borders that have little to do with underlying social/ethnic/whatever borders. As a result, you have a lot of ethnicity driven/motivated/aligned conflicts (granted, these conflicts tend to have multiple root causes, but surely ethnicity is often pretty critical) that take place within a country, rather than between countries.