We tend to think of the aggressor when we say war is bad. It makes sense, they’re the ones who initiate the conflict and make the war exist.
Defending yourself in a war though is, well, defensible. Being anti war can never be an absolutist position. Otherwise, those who are fine with war only need threaten war to get what they want. Do you truly live in peace if it’s because you give the aggressor everything they want? I’d argue no.