The Children's Crusade is alluded to throughout the story, most notably in a discussion between Mary O'Hare and the narrator, Vonnegut, over the fictional book Vonnegut is writing. The Children's Crusade is also alluded to when the Colonel sees the young American soldiers. In both instances, World War II is compared to the Children's Crusade.
The Children's Crusade is described by Vonnegut as follows: "the Children’s Crusade started in 1213, when two monks got the idea of raising armies of children in Germany and France, and selling them in North Africa as slaves." The Children's Crusade was then an unjust attempt to profit off of selling gullible children into slavery. As such, comparing World War II to the Children's Crusade is an unambiguous condemnation of the war. While World War II is often glorified as a just war, for Vonnegut the soldiers are akin to gullible children who trusted the Church and paid the ultimate price because of it.
The full title of Slaughterhouse-Five is Slaughterhouse-Five, or, The Children's Crusade: A Duty-Dance with Death. The allusion to the Children's Crusade is then highly significant, and Vonnegut's stance on both World War II specifically and war more generally is clear from the title alone.
At the end of the first chapter, there are two allusions to the biblical Book of Genesis:
And Lot’s wife, of course, was told not to look back where all those people and their homes had been. But she did look back, and I love her for that, because it was so human. So she was turned to a pillar of salt. So it goes. [...]
I’ve finished my war book now. The next one I write is going to be fun. This one is a failure, and had to be, since it was written by a pillar of salt.
The Book of Genesis explains that angels were going to punish the people of Sodom but wanted to spare Lot's family. Because Lot's wife turned and looked back, attempting to see the destruction wrought by the angels, she was turned into a pillar of salt. The narrator claims to love Lot's wife because to look back was human. For Vonnegut, then, to make mistakes, and to die as a result of those mistakes, is to be human. Considering the story centers around World War II, this humanizing message is quite relevant.
There is also a metaphor in the quotation above, as the narrator claims the story of Billy Pilgrim was written by a pillar of salt. The narrator, a stand-in for Vonnegut, then compares himself to the pillar of salt from Genesis: through reflecting and writing on his time as a soldier, he is constantly 'looking back' at destruction, and is punished accordingly. Vonnegut, by writing this book, is once again bearing witness to the horrors of war. The allusion to Lot's wife than reveals what Vonnegut thinks of himself as the author of Slaughterhouse-Five: a flawed human who has made and will make mistakes, complicit both as a real-life solider who fought in World War II and as voyeur in the horrors that he witnessed. The themes of witness and mercy, as well as war and destruction, are both furthered at the end of the first chapter through the allusion to the Book of Genesis.
Billy Pilgrim takes the place of Cinderella in a moment of situational irony in Chapter 6:
The boots fit perfectly. Billy Pilgrim was Cinderella, and Cinderella was Billy Pilgrim.
The scene is an allusion to fairy tale of Cinderella, which was just performed by the English prisoners of war. In the fairy tale, Prince Charming identified Cinderella through the fit of her shoe, and Vonnegut is playfully referencing that moment as Billy finally finds a new pair of shoes that fit him. The moment is situationally ironic because Billy Pilgrim is currently a prisoner of war in enemy territory. He is, in other words, in a terrible situation, a far cry from being a princess swept off of her feet by a charming prince. The sheer absurdity of the reference makes it darkly humorous.
And yet, Pilgrim is in some ways chosen by fate. He manages to survive the war against all odds, when most of his fellow soldiers are killed in one way or another. He was also the lone survivor of a plane crash. Cinderella, through the help of some magic, is saved from her destitute life against all odds; similarly, things just seem to work out for Billy Pilgrim, no matter how life-threatening the situation appears. While an ironic comparison, the Cinderella allusion highlights Billy Pilgrim's unlikely fate in an amusing way.
The narrator makes use of both a simile and an allusion to the fictional city of Oz when describing what Dresden looked like when seeing it for the first time:
The boxcar doors were opened, and the doorways framed the loveliest city that most of the Americans had ever seen. The skyline was intricate and voluptuous and enchanted and absurd. It looked like a Sunday school picture of Heaven to Billy Pilgrim. Somebody behind him in the boxcar said, “Oz.” That was I. That was me. The only other city I’d ever seen was Indianapolis, Indiana.
The city can barely be described through Billy Pilgrim's eyes: with every word he uses he has to use yet another to attempt to capture the wonder of the city, making Dresden "intricate and voluptuous and enchanted and absurd." The simile comparing Dresden to heaven suggests the city is akin to the kingdom of God, amongst the highest praise one could give. This conveys the sheer awe felt by the Americans who are viewing Dresden for the first time. The reader, however, knows Dresden will soon be burned down, and tens of thousands will be killed in the process. If Dresden is akin to heaven, the bombing of Dresden is an act of sin. Once more, the novel's anti-war sentiment is made apparent through figurative language.
The allusion to the fictional city of Oz (from the well-known book and film The Wizard of Oz) further conveys the wonder of Dresden, especially because Vonnegut inserts himself into the narrative to do so. Not only is Dresden akin to the magical city of Oz and Heaven, but it is so awe-inspiring that the narrator himself must enter the story to support the claim. Considering that Dresden is about to be destroyed, both the simile and the allusion only work to underscore the tragedy that is its impending destruction. By highlighting the wonder of Dresden through figurative language, Vonnegut ultimately indicts the collateral damage of war.