The Purity War was, according to David in Allegiant, "A civil war, waged by those with damaged genes, against the government and everyone with pure genes." After the war was over, many demanded a solution to the problem; for this reason, the Bureau of Genetic Welfare was formed.
It was implied the the Purity War was highly destructive, for it was said to have eliminated half the population of North America—"a level of destruction formerly unheard of on American soil".