The only time a war would be justifiable, besides being attacked and going on D, would probably be a threat trowards one country.

Lets say country A has some kind of powerful weaponry, and issues threats to use on country B if they do not follow commands issued by A.

In my opinion that would be a justifiable reason for Country b to go to war with Country A.

(A and B only used to not start some kind of conspiracy...)

It might be for the defense of ones country., but it's still taking the offensive.