Yes, destroying civilian targets with no valid military usage is considered a war crime. Destroying a civilian building being used as a military facility, on t

Yes, destroying civilian targets with no valid military usage is considered a war crime. Destroying a civilian building being used as a military facility, on the other hand, is not a war crime, except if there was no valid military purpose to the strike or the strike was not proportional to the military value that might be gained (e.g. it would never be acceptable post-Geneva to kill 1000 civilians just to destroy an enemy soldier). That was one of the justifications the Germans used to sink Lusitania (which despite American and British protestations, it appears the Germans had actually been right in retrospect).