Answer: I would make such a differentiation: a justified war is a war of people fighting for their existence and for their territory, for their land. If somebody comes to them and wants to kill them, then as it is written: “forestall by slaying him.” (Tractate Sanhedrin, 72:1, Gemarah)
On the other hand, a country shouldn’t be going to war when it is not on its own territory, even if it does so under certain “humanitarian” guises. It should only use help from international powers when seeking peace and some kind of dialog, but not for war and military operations.
Therefore, only one thing is justified: If someone from the outside rises against a person, or a group of people, or a whole country, then in response, you can fight him and kill him, or even start this war first. However, if nobody from the outside rose against you, you should not attack him and fight with him under any pretext, even the most incredible one.
The Round Table of Independent Opinions, Berlin 9/9/06