A "country's interests" are not that of most of their population. Okay, gray and fuzzy area: When most citizens of European countries where enthusiastic when WWI started, does that mean that war was in their interest? So let's add the condition that you are not under the influence of propaganda and group-think and ha time to assess and feel the effects of policies. It's still fuzzy though: If you, directly or indirectly, depend on income streams that flow through the military–industrial complex, is war in your interest if you yourself are not negatively impacted?