What you seem to be missing is that War is a macro-aggressive, acute failure of society. Microaggression is a stealthy, sinister, chronic failure of society that is far more widespread and far more damaging to the long-term health of humanity than is an acute War that has a beginning and an end.
Others have addressed the first major flaw in this argument, which is that killing people is worse than being mean to them.
But there's another flaw, which is your apparent belief that microaggression is something new. It is definitely not. People have always been nasty to each other, and we're significantly less nasty to each other today than ever before. The notion of microaggression is perhaps the best proof: previous generations didn't even bother thinking about microaggression, because it was just normal. Today, we recognize this subtle form of personal attack and work to expose it and thereby reduce it.
You should read the first few chapters of Steven Pinker's "The Better Angels of Our Nature", in which he documents historical evidence of the ways in which people were nasty to each other. He focuses mostly on physical nastiness, violence, but lots of other sorts of nastiness are covered in passing, or obviously implied. Society is much, much better than it used to be. Empathy for strangers is normal today. It wasn't always.