With a few exceptions wars have only killed off a vanishingly small portion of the population to cause much of genetic impact. Any impact it has will pale in comparison to the effect of the tendancy that those with a PHD tend to have far less children then those who dropped out of high school.
That brings up another subject. With the invention of the pill in the 60s, double X chromosome intelligence has become a self-limiting trait. I don't think men with Ph.Ds father significantly less children. The pill hasn't been around long enough to have had a major genetic impact but it will be significant in the future. Before the pill, highly educated women had many children.
If you don't think wars have had a significant genetic impact, look around. How many Native North Americans do you see? Thinking that world wars have ended is a bad bet. It's just a matter of time before the terrorists get a nuclear device and kill millions of people, setting off another revision to the gene pool.