In Enders Game the effects of what appear to be even the most harmless games is very real death and destruction. People tend to dismiss games as, well, fun and games, but in the novel the games themselves cause far more damage than anything else. The Poem The Game of War by Aaron Tate is similar to the intent of the novel.
In the beginning of the novel Ender and his brother Peter play a game known as buggers and astronauts. Even though it is just a game the incredibly brutal Peter uses it as an opportunity to inflict very real damage onto his brother. Because this game is a pretend fight between the heroic human marines and the evil aliens it fits in literally with the tile of the poem "The Game of War", "buggers versus astronauts was a "game of war".
The central theme of the battle school is the battle room where the boys play a game in which infantry maneuvers and engagements are simulated.
The competition propagated throughout the school by this game causes the death of one boy in the end. The boy, called Bonzo, is so angry that he comes for Ender with several of his friends saying that "Nobody came to fight you; we just came to talk to you about playing fair with the games. Maybe lose a couple now and then", despite his claim that no one came to fight, there is a fight in which Ender kills Bonzo. Which parallels the line "War has envy" from TGW because Bonzo had come to fight Ender out of envy.
Finally at Command School Ender and his peers play what they believe to be a fleet combat simulation but in the end it turns out that they were actually directing real fleets to...