https://www.youtube.com/watch?v=24XKJSBNrPg
Attack on Titan is an action, post-apocalyptic anime and manga series by Hajime Isayama. The setting is within the walls of Maria, Rose, and Sina where the characters live in fear of titans, giant humanoid creatures whose diets consist of humans. In the beginning, it is believed that the people within the walls are the last humans left.
"Boku No Sensou" is the first opening of the final season where the characters discover that humanity has thrived beyond the walls and that titans are actually people that have been transformed. A few reccuring themes in Attack on Titan are the repetition of history. The wall people (Eldians) are targeted for their participation in a previous war hundreds of years ago. Their defeat in the war was their fall from grace. It's a cycle of violence that was neverending. The characters had originally hoped to thrive as the human race once again, only to fall into the hands of another war, then back to hoping to return to good times once again.
Within The Waste Land, we have a sense of time repeating. "Breeding lilacs out of the dead land" set us in a barren land. However, we must return back to the key term "breeding." April is described as cruel for bringing the lilacs back to the waste land. Springtime is a time of change and new beginnings but it doesn't last forever. The cruelty can come out when these flowers are once again ripped away. Then the cycle begins anew, spring, to summer, to autumn, to winter, back to spring.
"I purified my imperfect flowers / The pain in my heart getting higher" from Boku No Sensou is similar to the breeding lilacs. Purification tends to have a very positive connotation where you get rid of all the bad things in something. However, the next line still shows pain is present, not only present but getting more persistent.
The lines of The Burial of the Dead and "Boku No Sensou" are connected through this cycle of change and stagnation. The flowers can continue to grow, but they'll still die at the end of the day. There is cruelty in change if things don't actually end up changing the way you want it to.