First, war is wrong for the same reasons that murder is wrong. Because war is murder.
A strategy that kills ten thousand people is not somehow less wrong than killing just one person. The colossal scale does not make it noble. The idea that countries rather than people are the “agents” is also irrelevant. Someone gave the order to go over there and start shooting, and if that person caused ten thousand people to die that’s ten thousand times as criminal as someone who caused only one person to die.
Second, war is the probably the worst possible solution to whatever problem you’re trying to solve. It’s worse than wrong, it’s idiotic.
Nothing broken can be fixed by killing people. In the future when someone suggests a war others will look away in embarrassment. And one of them will say: “I’m sorry, but that’s just stupid. Why don’t you go off and think for a while about exactly what the problem is that you’re trying to solve. When you come back, we can collaborate on ways to fix that one thing instead of breaking twelve others.”