Symbolism, Imagery, Allegory
You will not find the word "Holocaust" in The History of Love, but the atrocities of World War II hang heavily over the novel anyway. Perhaps the most overt reference in the novel is a brief passage in which Alma considers the various "mass extinctions" throughout planetary history. She writes bluntly (in a line standing as its own separate paragraph), "I did a search on mass extinctions" (8.14), a reference to the way her search for Alma Mereminski leads her back to events in Poland sixty years prior.
In that search, though, Alma is specifically thinking about butterflies, which happen to be a symbol of the fragility of human life. (Elsewhere in the novel, Leo recalls a time when his Alma accidentally crushed a moth she held in her hands.) Alma notes that the current annihilation of species "isn't caused by natural events, but by the ignorance of human beings" (8.15).